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Lakhmir Singh solutions for Class 9 Science chapter 3 - Atoms and Molecules

Class 9 Chemistry - Science Part 2

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Lakhmir Singh Class 9 Chemistry - Science Part 2

Class 9 Chemistry - Science Part 2 - Shaalaa.com

Chapter 3: Atoms and Molecules

Very Short AnswersShort AnswersLong AnswersMultiple Choice QuestionsOthersHots Questions

Chapter 3: Atoms and Molecules Exercise Very Short Answers, Short Answers, Long Answers, Multiple Choice Questions, Hots Questions solutions [Pages 128 - 132]

Very Short Answers | Q 1 | Page 128

Write the full form of IUPAC.

Very Short Answers | Q 2.1 | Page 128

Name the scientist who gave law of conservation of mass.

Very Short Answers | Q 2.2 | Page 128

Name the scientist who gave law of constant proportions.

Very Short Answers | Q 3.1 | Page 128

Name the law of chemical combination which was given by Lavoisier.

Very Short Answers | Q 3.2 | Page 128

Name the law of chemical combination which was given by Proust.

Very Short Answers | Q 4 | Page 129

Name the scientist who gave atomic theory of matter.

Very Short Answers | Q 5 | Page 129

Which postulate of Dalton's atomic theory is the result of law of conservation of mass given by Lavoisier ?

Very Short Answers | Q 6 | Page 129

Which part of the Dalton's atomic theory came from the law of constant proportions given by Proust ?

Very Short Answers | Q 7 | Page 129

Which ancient Indian philosopher suggested that all matter is composed of very small particles ? What
name was given by him to these particles ?

Very Short Answers | Q 8 | Page 129

Name any two laws of chemical combination.

Very Short Answers | Q 9 | Page 129

‘If 100 grams of pure water taken from different sources is decomposed by passing electricity, 11 grams of hydrogen and 89 grams of oxygen are always obtained’. Which chemical law is illustrated by this statement ?

Very Short Answers | Q 10 | Page 129

‘If 100 grams of calcium carbonate (whether in the form of marble or chalk) are decomposed completely, then 56 grams of calcium oxide and 44 grams of carbon dioxide are obtained’. Which law of chemical combination is illustrated by this statement ?

Very Short Answers | Q 11 | Page 129

What are the building blocks of matter ?

Very Short Answers | Q 12 | Page 129

How is the size of an atom indicated ?

Very Short Answers | Q 13 | Page 129

Name the unit in which the radius of an atom is usually expressed.

Very Short Answers | Q 14 | Page 129

Write the relation between nanometre and metre.

Very Short Answers | Q 15 | Page 129

The radius of an oxygen atom is 0.073 nm. What does the symbol ‘nm’ represent ?

Very Short Answers | Q 16 | Page 129

Why is it not possible to see an atom even with the most powerful microscope ?

Very Short Answers | Q 17 | Page 129

State whether the following statement is true or false:

The symbol of element cobalt is CO.

Very Short Answers | Q 18 | Page 129

Define 'molecular mass' of a substance.

Very Short Answers | Q 19 | Page 129

What is meant by saying that ‘the molecular mass of oxygen is 32’ ?

Very Short Answers | Q 20.1 | Page 129

Fill in the following blanks with suitable words :

In water, the proportion of oxygen and hydrogen is .............. by mass.

Very Short Answers | Q 20.2 | Page 129

Fill the following blanks with suitable words :

In a chemical reaction, the sum of the masses of the reactants and the products remains unchanged. This is called ....................

Short Answers | Q 21.1 | Page 129

Name the element used as a standard for atomic mass scale.

Very Short Answers | Q 21.2 | Page 129

Which particular atom of the above element is used for this purpose ?

Short Answers | Q 21.3 | Page 129

What value has been given to the mass of this reference atom ?

Very Short Answers | Q 22 | Page 129

Give one major drawback of Dalton's atomic theory of matter.

Short Answers | Q 23 | Page 129

Dalton's atomic theory says that atoms are indivisible. Is this statement still valid ? Give reasons for your answer.

Short Answers | Q 24 | Page 129

Is it possible to see atoms these days ? Explain your answer.

Short Answers | Q 25 | Page 129

What is meant by the symbol of an element ? Explain with examples.

Short Answers | Q 26.1 | Page 129

Give two symbols which have been derived from the "English names" of the elements.

Short Answers | Q 26.2 | Page 129

Give two symbols which have been derived from the " Latin names" of the elements.

Short Answers | Q 27 | Page 129

Give the names and symbols of five familiar substances which you think are elements.

Short Answers | Q 28 | Page 129

State the chemical symbols for the following elements :

Sodium, Potassium, Iron, Copper, Mercury, Silver.

Short Answers | Q 29 | Page 129

Name the elements represented by the following symbols :

Hg, Pb, Au, Ag, Sn

Short Answers | Q 30 | Page 129

What is meant by atomicity ? Explain with two examples.

Short Answers | Q 31 | Page 129

What is the atomicity of the following ?

  1. Oxygen
  2. Ozone
  3. Neon
  4. Sulphur
  5. Phosphorus
  6. Sodium
Short Answers | Q 32 | Page 129

What is meant by a chemical formula ? Write the formulae of one element and one compound.

Short Answers | Q 33 | Page 129

Write the formulae of the following compounds. Also name the elements present in them.

  1. Water
  2. Ammonia
  3. Methane
  4. Sulphur dioxide
  5. Ethanol
Short Answers | Q 34 | Page 129

Explain the difference between 2N and N2.

Short Answers | Q 35 | Page 129

What do the following abbreviations stand for ?

  1. O
  2. 2O
  3. O2
  4. 3O2
Short Answers | Q 36 | Page 130

What do the symbols, H2, S and O4 mean in the formula H2SO4 ?

Short Answers | Q 37.1 | Page 130

In what form does oxygen gas occur in nature ?

Short Answers | Q 37.2 | Page 130

In what form do noble gases occur in nature ?

Short Answers | Q 38 | Page 130

What is the difference between 2H and H2 ?

Short Answers | Q 39 | Page 130

What do the following denote ?

  1. N
  2. 2N
  3. N2
  4. 2N2
Short Answers | Q 40 | Page 130

What is the significance of the formula of a substance ?

Short Answers | Q 41 | Page 130

What is the significance of the formula H2O ?

Short Answers | Q 42 | Page 130

The molecular formula of glucose is C6H12O6. Calculate its molecular mass. (Atomic masses : C = 12 u ; H = 1 u ; O = 16 u)

Short Answers | Q 43.1 | Page 130

Calculate the molecular masses of the following :

  1. Hydrogen, H2

(Atomic masses : H = 1 u ; O = 16 u ; Cl = 35.5 u ; N = 14 u ; C = 12 u)

Short Answers | Q 43.2 | Page 130

Calculate the molecular masses of the following :

Oxygen, O2

(Atomic masses : H = 1 u ; O = 16 u ; Cl = 35.5 u ; N = 14 u ; C = 12 u)

Short Answers | Q 43.3 | Page 130

Calculate the molecular masses of the following :

Chlorine, Cl2

(Atomic masses : H = 1 u ; O = 16 u ; Cl = 35.5 u ; N = 14 u ; C = 12 u)

Short Answers | Q 43.4 | Page 130

Calculate the molecular masses of the following :

Ammonia, NH3

(Atomic masses : H = 1 u ; O = 16 u ; Cl = 35.5 u ; N = 14 u ; C = 12 u)

Short Answers | Q 43.5 | Page 130

Calculate the molecular masses of the following :

Carbon dioxide, CO2

(Atomic masses : H = 1 u ; O = 16 u ; Cl = 35.5 u ; N = 14 u ; C = 12 u)

Short Answers | Q 44.1 | Page 130

Calculate the molecular masses of the following compounds  :

Methane, CH4

(Atomic masses : C = 12 u ; H = 1 u)

Short Answers | Q 44.2 | Page 130

Calculate the molecular masses of the following compounds  :

Ethane, C2H

(Atomic masses : C = 12 u ; H = 1 u)

Short Answers | Q 44.3 | Page 130

Calculate the molecular masses of the following compounds  :

Ethene, C2H

(Atomic masses : C = 12 u ; H = 1 u)

Short Answers | Q 44.4 | Page 130

Calculate the molecular masses of the following compounds  :

Ethyne, C2H2

(Atomic masses : C = 12 u ; H = 1 u)

Short Answers | Q 45.1 | Page 130

Calculate the molecular masses of the following compounds :

Methanol, CH3OH

Short Answers | Q 45.2 | Page 130

Calculate the molecular masses of the following compounds :

Ethanol, C2H5OH

Short Answers | Q 46 | Page 130

Calculate the molecular mass of ethanoic acid, CH3COOH.
(Atomic masses : C = 12 u ; H = 1 u ; O = 16 u)

Short Answers | Q 47 | Page 130

Calculate the molecular mass of nitric acid, HNO3. (Atomic masses : H = 1 u ; N = 14 u ; O = 16 u)

Short Answers | Q 48 | Page 130

Calculate the molecular mass of chloroform (CHCl3). (Atomic masses : C = 12 u ; H = 1 u ; Cl = 35.5 u)

Short Answers | Q 49 | Page 130

Calculate the molecular mass of hydrogen bromide (HBr). (Atomic masses : H = 1 u ; Br = 80 u)

Short Answers | Q 50.1 | Page 130

Calculate the molecular masses of the following compounds :

Hydrogen sulphide, H2S

Short Answers | Q 50.2 | Page 130

Calculate the molecular masses of the following compounds :

Carbon disulphide, CS2

(Atomic masses : H = 1 u ; S = 32 u ; C = 12 u)

Long Answers | Q 51 | Page 130

State the law of conservation of mass. Give one example to illustrate this law.

Long Answers | Q 52 | Page 130

State the law of constant proportions. Give one example to illustrate this law.

Long Answers | Q 53.1 | Page 130

State the various postulates of Dalton’s atomic theory of matter.

Long Answers | Q 53.2 | Page 130

Which postulate of Dalton's atomic theory can explain the law of conservation of mass ?

Long Answers | Q 53.3 | Page 130

Which postulate of Dalton’s atomic theory can explain the law of constant proportions ?

Long Answers | Q 54.1 | Page 130

What is the significance of the symbol of an element ? Explain with the help of an example.

Long Answers | Q 54.2 | Page 130

Explain the significance of the symbol H.

Long Answers | Q 55.1 | Page 130

What is an atom ? How do atoms usually exist ?

Long Answers | Q 55.2 | Page 130

What is a molecule ? Explain with an example.

Long Answers | Q 55.3 | Page 130

What is the difference between the molecule of an element and the molecule of a compound ? Give one
example of each.

Long Answers | Q 56.1 | Page 130

Define atomic mass unit. What is its symbol ?

Long Answers | Q 56.2 | Page 130

Define atomic mass of an element.

Long Answers | Q 56.3 | Page 130

What is meant by saying that 'the atomic mass of oxygen is 16' ?

Multiple Choice Questions | Q 57 | Page 130

The atomicities of ozone, sulphur, phosphorus and argon are respectively :

  • 8, 3, 4 and 1

  • 1, 3, 4 and 8

  • 4, 1, 8 and 3

  • 3, 8, 4 and 1

Multiple Choice Questions | Q 58 | Page 130

The symbol of a metal element which is used in making thermometers is :

  • Ag

  • Hg

  • Mg

  • Sg

Multiple Choice Questions | Q 59 | Page 131

The Latin language name of an element is natrium. The English name of this element is :

  • sodium

  • potassium

  • magnesium

  • sulphur

Multiple Choice Questions | Q 60 | Page 131

The atomic theory of matter was proposed by :

  • John Kennedy

  • Lavoisier

  • Proust

  • John Dalton

Multiple Choice Questions | Q 61 | Page 131

One of the following elements has an atomicity of ‘one’. This element is :

  • helium

  • hydrogen

  • sulphur

  • ozone

Multiple Choice Questions | Q 62 | Page 131

The English name of an element is potassium, its Latin name will be :

  • plumbum

  • cuprum

  • kalium

  • natrium

Multiple Choice Questions | Q 63 | Page 131

The law of conservation of mass was given by :

  • Dalton

  • Proust

  • Lavoisier

  • Berzelius

Multiple Choice Questions | Q 64 | Page 131

The element having atomicity 'four' is most likely to be :

  • argon

  • fluorine

  • phosphorus

  • francium

Multiple Choice Questions | Q 65 | Page 131

If 1.4 g of calcium oxide is formed by the complete decomposition of calcium carbonate, then the amount of
calcium carbonate taken and the amount of carbon dioxide formed will be respectively :

  • 2.2 g and 1.1 g

  • 1.1 g and 2.5 g

  • 2.5 g and 1.1 g

  • 5.0 g and 1.1 g

Multiple Choice Questions | Q 66 | Page 131

The law of constant proportions was given by :

  • Proust

  • Lavoisier

  • Dalton

  • Berzelius

Q 67 | Page 131

Out of ozone, phosphorus, sulphur and krypton, the elements having the lowest and highest atomicities are respectively :

  • sulphur and krypton

  • krypton and ozone

  • phosphorus and sulphur

  • krypton and sulphur

Multiple Choice Questions | Q 68 | Page 131

One nm is equal to :

  • 10–9 mm

  • 10–7 cm

  • 10–9 cm

  • 10–6 m

Multiple Choice Questions | Q 69 | Page 131

The scientist who proposed the first letter (or first letter and another letter) of the Latin or English name of
an element as its symbol, was :

  • Dalton

  • Proust

  • Lavoisier

  • Berzelius

Multiple Choice Questions | Q 70 | Page 131

The atoms of which of the following pair of elements are most likely to exist in free state ?

  • hydrogen and helium

  • argon and carbon

  • neon and nitrogen

  • helium and neon

Multiple Choice Questions | Q 71 | Page 131

Which of the following elements has the same molecular mass as its atomic mass ?

  • nitrogen

  • neon

  • oxygen

  • chlorine

Multiple Choice Questions | Q 72 | Page 131

In water, the proportion of oxygen and hydrogen by mass is :

  • 1 : 4

  • 1 : 8

  • 4 : 1

  • 8 : 1

Multiple Choice Questions | Q 73 | Page 131

In hydrogen peroxide (H2O2), the proportion of hydrogen and oxygen by mass is :

  • 1 : 8

  • 1 : 16

  • 8 : 1

  • 16 : 1

Multiple Choice Questions | Q 74 | Page 131

The symbols of the elements cobalt, aluminium, helium and sodium respectively written by a student are as
follows. Which symbol is the correct one ?

  • CO

  • AL

  • He

  • So

Hots Questions | Q 75 | Page 131

Copper sulphate reacts with sodium hydroxide to form a blue precipitate of copper hydroxide and sodium
sulphate. In an experiment, 15.95 g of copper sulphate reacted with 8.0 g of sodium hydroxide to form
9.75 g of copper hydroxide and 14.2 g of sodium sulphate. Which law of chemical combination is illustrated by this data ? Give reason for your choice.

Hots Questions | Q 76 | Page 131

Potassium chlorate decomposes, on heating, to form potassium chloride and oxygen. When 24.5 g of potassium chlorate is decomposed completely, then 14.9 g of potassium chloride is formed. Calculate the mass of oxygen formed. Which law of chemical combination have you used in solving this problem ?

Hots Questions | Q 77 | Page 131

In an experiment, 4.90 g of copper oxide was obtained from 3.92 g of copper. In another experiment, 4.55 g of copper oxide gave, on reduction, 3.64 g of copper. Show with the help of calculations that these figures verify the law of constant proportions.

Hots Questions | Q 78 | Page 131

Magnesium and oxygen combine in the ratio of 3 : 2 by mass to form magnesium oxide. What mass of oxygen gas would be required to react completely with 24 g of magnesium ?

Hots Questions | Q 79 | Page 132

When 5 g of calcium is burnt in 2 g of oxygen, then 7 g of calcium oxide is produced. What mass of calcium oxide will be produced when 5 g of calcium is burnt in 20 g of oxygen ? Which law of chemical combination will govern your answer ?

Hots Questions | Q 80 | Page 132

A liquid compound X of molecular mass 18 u can be obtained from a number of natural sources. All the animals and plants need liquid X for their survival. When an electric current is passed through 200 grams of pure liquid X under suitable conditions, then 178 grams of gas Y and 22 grams of gas Z are produced. Gas Y is produced at the positive electrode whereas gas Z is obtained at the negative electrode. Moreover, gas Y supports combustion whereas gas Z burns itself causing explosions.

  1. Name (i) liquid X (ii) gas Y, and (iii) gas Z.
  2. What is the ratio of the mass of element Z to the mass of element Y in the liquid X ?
  3. Which law of chemical combination is illustrated by this example ?
  4. Name two sources of liquid X.
  5. State an important use of Y in our life.
Hots Questions | Q 81 | Page 131

One of the forms of a naturally occurring solid compound P is usually used for making the floors of houses. On adding a few drops of dilute hydrochloric acid to P, brisk effervescence are produced. When 50 g of reactant P was heated strongly, than 22 g of a gas Q and 28 g of a solid R were produced as products. Gas Q is the same which produced brisk effervescence on adding dilute HCl to P. Gas Q is said to cause global warming whereas solid R is used for white-washing.

  1. What is (i) solid P (ii) gas Q, and (iii) solid R.
  2. What is the total mass of Q and R obtained from 50 g of P ?
  3. How does the total mass of Q and R formed compare with the mass of P taken ?
  4. What conclusion do you get from the comparison of masses of products and reactant ?
  5. Which law of chemical combination is illustrated by the example given in this problem ?

Chapter 3: Atoms and Molecules Exercise Very Short Answers, Short Answers, Long Answers, Multiple Choice Questions, Hots Questions solutions [Pages 150 - 153]

Very Short Answers | Q 1 | Page 150

What do we call those particles which have more or less electrons than the normal atoms ?

Very Short Answers | Q 2.1 | Page 150

What do we call those particles which have more electrons than the normal atoms ?

Very Short Answers | Q 2.2 | Page 150

What do we call those particles which have less electrons than the normal atoms ?

Very Short Answers | Q 3 | Page 150

Define ‘formula mass’ of a compound.

Very Short Answers | Q 4.1 | Page 151

What do we call those particles which are formed by the gain of electrons by atoms ?

Very Short Answers | Q 4.2 | Page 151

What do we call those particles which are formed by the loss of electrons by atoms ?

Very Short Answers | Q 5.1 | Page 151

State whether the following statements are true or false:

A sodium ion has positive charge because it has more protons than a neutral atom.

Very Short Answers | Q 5.2 | Page 151

State whether the following statements are true or false :

A chloride ion has negative charge because it has more electrons than a neutral atom.

Very Short Answers | Q 6.1 | Page 151

An element Z has a valency of 3. What is the formula of oxide of Z ?

Very Short Answers | Q 6.1 | Page 151

Write down the formulae for the following compounds :

Calcium oxide

Very Short Answers | Q 6.2 | Page 150

Write down the formulae for the following compounds :

Magnesium hydroxide

Very Short Answers | Q 7 | Page 151

An element Z has a valency of 3. What is the formula of oxide of Z ?

Very Short Answers | Q 8 | Page 151

What is the name of a particle which contains 10 electrons, 11 protons and 12 neutrons ?

Very Short Answers | Q 9 | Page 151

Name the particle which has 18 electrons, 18 neutrons and 17 protons in it.

Very Short Answers | Q 10.1 | Page 151

Fill in the following blank with suitable word:

The particle which is formed by the loss or gain of electrons by an atom is called..............

Very Short Answers | Q 10.2 | Page 151

Fill in the following blank with suitable word:

The particle which is formed by the loss of electrons by an atom is called..............

Very Short Answers | Q 10.3 | Page 151

Fill in the following blank with suitable word :

The particle which is formed by the gain of electrons by an atom is called ..............

Very Short Answers | Q 10.4 | Page 151

Fill in the following blank with suitable word:

A potassium ion has positive charge because it contains less............... than.....................

Very Short Answers | Q 10.5 | Page 151

Fill in the following blank with suitable word:

A sulphide ion has negative charge because it contains less ....................... than ...................

Short Answers | Q 11 | Page 151

Name the elements water is made of. What are the valencies of these elements ? Work out the chemical
formula for water.

Short Answers | Q 12 | Page 151

If the valency of hydrogen is 1 and that of nitrogen is 3, work out the formula for ammonia.

Short Answers | Q 13 | Page 151

Work out the formula for sulphur dioxide. (Valencies : S = 4 ; O = 2)

Short Answers | Q 14 | Page 151

If the valency of carbon is 4 and that of sulphur is 2, work out the formula of the compound formed by the combination of carbon with sulphur. What is the name of this compound ?

Short Answers | Q 15 | Page 151

An element X has a valency of 4 whereas  another element Y has a valency of 1. What will be the formula of the compound formed between X and Y ?

Short Answers | Q 16 | Page 151

An element B shows valencies of 4 and 6. Write the formulae of its two oxides.

Short Answers | Q 17 | Page 151

An element X of valency 3 combines with another element Y of valency 2. What will be the formula of the compound formed ?

Short Answers | Q 18 | Page 151

Work out the formula for magnesium hydrogencarbonate.

Short Answers | Q 19.1 | Page 151

An element X has a valency of 2. Write the simplest formula for bromide of the element.

Short Answers | Q 19.2 | Page 151

An element X has a valency of 2. Write the simplest formula for oxide of the element.

Short Answers | Q 20.1 | Page 151

Work out the formulae for the following compounds :

Sodium oxide

Short Answers | Q 20.2 | Page 151

Work out the formulae for the following compounds:

Calcium carbonate

Short Answers | Q 21.1 | Page 151

Calculate the formula masses of the following compounds :

Sodium oxide, Na2O

Short Answers | Q 21.2 | Page 151

Calculate the formula masses of the following compounds :

Calcium carbonate

Short Answers | Q 22 | Page 151

Name the following compounds. Also write the symbols/formulae of the ions present in them :

  1. CuSO4
  2. (NH4)2SO4
  3. Na2O
  4. Na2CO3
  5. CaCl2
Short Answers | Q 23 | Page 151

Write the cations and anions present, if any, in the following :

  1. CH3COONa
  2. NaCl
  3. H2
  4. NH4NO3
Short Answers | Q 24 | Page 151

Give the formulae of the compounds formed from the following sets of elements :

  1. calcium and fluorine
  2. hydrogen and sulphur
  3. nitrogen and hydrogen
  4. carbon and chlorine
  5. sodium and oxygen
  6. carbon and oxygen
Short Answers | Q 25 | Page 151

What are (i) ionic compounds, and (ii) molecular compounds ? Give two examples of each type of compounds.

Long Answers | Q 26.1 | Page 152

What is an ion ? How is an ion formed ? Explain with the help of two examples of different ions.

Long Answers | Q 26.2 | Page 152

The valencies (or charges) of some of the ions are given below : 

Ion Valency (Charge) Ion Valency (Charge)
Sodium ion 1+ Bromide ion 1–
Ammonium ion 1+ Hydroxide ion 1–
Calcium ion 2+ Sulphate ion 2–
Lead ion 2+ Phosphate ion 3–


Using this information, write down the formulae of the following compounds :
(i) Sodium phosphate
(ii) Ammonium sulphate
(iii) Calcium hydroxide
(iv) Lead bromide 

Long Answers | Q 27.1 | Page 152

What is the difference between a cation and an anion ? Explain with examples.

Long Answers | Q 27.2 | Page 152

The valencies (or charges) of some of the ions are given below :

Ion            Valency        Ion                Valency                        (Charge)                             (Charge)

Sodium ion       1+      Nitrate ion            1–  
Copper ion       2+       Sulphide ion         2–

Using this information, write down the formulae of :

  1. Sodium sulphide
  2. Copper nitrate
Long Answers | Q 28 | Page 152

Explain the formation of (i) sodium ion, and (ii) chloride ion, from their respective atoms giving the number of protons and number of electrons in each one of them. What is the reason for positive charge on a sodium ion and a negative charge on a chloride ion ?

Long Answers | Q 29.1 | Page 152

(a) Write the symbols/formulae of two simple ions and two compound ions (or polyatomic ions).

Long Answers | Q 29.2 | Page 152

An element Y has a valency of 4. Write the formula for its :

  1. chloride
  2. oxide
  3. sulphate
  4. carbonate
  5. nitrate
Long Answers | Q 30 | Page 152

Define ‘formula unit’ of an ionic compound. What is the formula unit of (i) sodium chloride, and
(ii) magnesium chloride ?

Long Answers | Q 30.2 | Page 152

Calculate the formula masses of the following compounds :

  1. Calcium chloride
  2. Sodium carbonate

(Given : Atomic masses : Ca = 40 u ; Cl = 35.5 u ; Na = 23 u ; C = 12 u; O = 16 u)

Multiple Choice Questions | Q 31 | Page 152

The atomic number of an element X is 13. What will be the number of electrons in its ion X3+ ?

  • 11

  • 15

  • 16

  • 10

Multiple Choice Questions | Q 32 | Page 152

Which of the following represents a correct chemical formula ?

  • CaCl

  • Na3N

  • NaSO4

  • NaS

Multiple Choice Questions | Q 33 | Page 152

If the number of electrons in an ion Z3– is 10, the atomic number of element Z will be :

  • 7

  • 5

  • 10

  • 8

Multiple Choice Questions | Q 34 | Page 152

The anion of an element has :

  • more electrons than the normal atom

  • less electrons than the normal atom

  • more protons than the normal atom

  • same number of electrons as normal atom

Multiple Choice Questions | Q 35 | Page 152

A particle X has 17 protons, 18 neutrons and 18 electrons. This particle is most likely to be :

  • a cation

  • an anion

  • a molecule

  • a compound

Multiple Choice Questions | Q 36 | Page 152

An element which can exhibit valencies of 2, 4 and 6 can be :

  • copper

  • iron

  • mercury

  • sulphur

Multiple Choice Questions | Q 37 | Page 153

The atomic number of an element E is 16. The number of electrons in its ion E2– will be :

  • 16

  • 18

  • 15

  • 14

Multiple Choice Questions | Q 38 | Page 153

The cation of an element has :

  • the same number of electrons as its neutral atom

  • more electrons than a neutral atom

  • less protons than a neutral atom

  • less electrons than a neutral atom

Multiple Choice Questions | Q 39 | Page 153

Two elements X and Y have valencies of 5 and 3, and 3 and 2, respectively. The elements X and Y are most likely to be respectively :

  • copper and sulphur

  • sulphur and iron

  • phosphorus and nitrogen

  • nitrogen and iron

Multiple Choice Questions | Q 40 | Page 153

The number of electrons in an ion Y2+ is 10. The atomic number of element Y is most likely to be :

  • 8

  • 12

  • 10

  • 14

Multiple Choice Questions | Q 41 | Page 153

A particle P has 18 electrons, 20 neutrons and 19 protons. This particle must be :

  • a molecule

  • a binary compound

  • an anion

  • a cation

Multiple Choice Questions | Q 42 | Page 153

An ionic compound will be formed by the combination of one of the following pairs of elements. This pair of elements is :

  • chlorine and calcium

  • calcium and sodium

  • sulphur and carbon

  • chlorine and chlorine

Multiple Choice Questions | Q 43 | Page 153

Molecular compounds are usually formed by the combination between :

  • a metal and a non-metal

  • two different non-metals

  • two different metals

  • any two gaseous elements

Multiple Choice Questions | Q 45 | Page 153

The formula of the sulphate of an element X is X2(SO4)3. The formula of nitride of element X will be :

  • X2N

  • XN2

  • XN

  • X2N3

Hots Questions | Q 46.1 | Page 153

An element A forms an oxide A2O5.

What is the valency of element A ?

Hots Questions | Q 46.2 | Page 153

An element A forms an oxide A2O5.

What will be the formula of chloride of A ?

Hots Questions | Q 47 | Page 153

An element X forms the following compounds with hydrogen, carbon and oxygen :

H2X, CX2, XO2, XO3

State the three valencies of element X which are illustrated by these compounds.

Hots Questions | Q 48 | Page 153

If the aluminium salt of an anion X is Al2X3, what is the valency of X ? What will be the formula of the magnesium salt of X ?

Hots Questions | Q 49.1 | Page 153

The formula of carbonate of a metal M is M2CO3.

What will be the formula of its iodide ?   

Hots Questions | Q 49.2 | Page 153

The formula of carbonate of a metal M is M2CO3.

What will be the formula of its nitride ?

Hots Questions | Q 49.3 | Page 153

The formula of carbonate of a metal M is M2CO3.

 What will be the formula of its phosphate ?

Hots Questions | Q 50 | Page 153

The atom of an element X contains 17 protons, 17 electrons and 18 neutrons whereas the atom of an element Y contains 11 protons, 11 electrons and 12 neutrons.

  1. What type of ion will be formed by an atom of element X ? Write the symbol of ion formed.
  2. What will be the number of (i) protons (ii) electrons, and (iii) neutrons, in the ion formed from X ?
  3. What type of ion will be formed by an atom of element Y ? Write the symbol of ion formed.
  4. What will be the number of (i) protons (ii) electrons, and (iii) neutrons, in the ion formed from Y ?
  5. What is the atomic mass of (i) X, and (ii) Y ?
  6. What could the elements X and Y be ?

Chapter 3: Atoms and Molecules Exercise Very Short Answers, Short Answers, Long Answers, Multiple Choice Questions, Hots Questions solutions [Pages 172 - 174]

Very Short Answers | Q 1 | Page 172

What is a group of 6.022 × 1023 particles known as ?

Very Short Answers | Q 2 | Page 172

What name is given to the amount of substance containing 6.022 × 1023 particles(atoms,molecules or ions) of a substance ?

Very Short Answers | Q 3 | Page 172

What is the numerical value of Avogadro number ?

Very Short Answers | Q 4 | Page 172

How many atoms are present in one gram atomic mass of a substance ?

Very Short Answers | Q 5 | Page 172

How many molecules are present in one gram molecular mass of a substance ?

Very Short Answers | Q 6 | Page 172

What name is given to the number 6.022 × 1023 ?

Very Short Answers | Q 7 | Page 172

Convert 12 g of oxygen gas into moles.

Very Short Answers | Q 8 | Page 172

How many moles are 3.6 g of water ?

Very Short Answers | Q 9 | Page 172

What is the mass of 0.2 mole of oxygen atoms ?

Very Short Answers | Q 10 | Page 172

Find the mass of 2 moles of nitrogen atoms.

Very Short Answers | Q 11.1 | Page 172

Fill in the following blanks :

1 mole contains .............. atoms, molecules or ions of a substance.

Very Short Answers | Q 11.2 | Page 172

Fill in the following blanks :

A mole represents an .............. number of particles of a substance.

Very Short Answers | Q 11.3 | Page 172

Fill in the following blanks :

60 g of carbon element are .............. moles of carbon atoms.

Very Short Answers | Q 11.4 | Page 172

Fill in the following blanks :

0.5 mole of calcium element has a mass of ............... .

Very Short Answers | Q 11.5 | Page 172

Fill in the following blanks :

64 g of oxygen gas contains ................ moles of oxygen atoms.

Short Answers | Q 12.1 | Page 172

How many atoms are there in exactly 12 g of carbon-12 element ? (C = 12 u)

Short Answers | Q 12.2 | Page 172

What name is given to this number ?

Short Answers | Q 12.3 | Page 172

What name is given to the amount of substance containing this number of atoms ?

Short Answers | Q 13 | Page 172

Calculate the mass of 12.044 × 1025 molecules of oxygen (O2).

Short Answers | Q 14 | Page 172

What is the number of molecules in 1.5 moles of ammonia ?

Short Answers | Q 15 | Page 172

How many moles of calcium carbonate (CaCO3) are present in 10 g of the substance ? (Ca = 40 u ; C = 12 u ; O = 16 u)

Short Answers | Q 16 | Page 172

How many moles of O2 are there in 1.20 × 1022 oxygen molecules ?

Short Answers | Q 17 | Page 172

If one mole of nitrogen molecules weighs 28 g, calculate the mass of one molecule of nitrogen in grams.

Short Answers | Q 18 | Page 172

How many moles are there in 34.5 g of sodium ? (Atomic mass of Na = 23 u)

Short Answers | Q 19 | Page 172

How many moles are there in 34.5 g of sodium ? (Atomic mass of Na = 23 u)

Short Answers | Q 20 | Page 172

Calculate the mass of 3.011 × 1024 atoms of carbon.

Short Answers | Q 21 | Page 172

If 16 g of oxygen contains 1 mole of oxygen atoms, calculate the mass of one atom of oxygen.

Short Answers | Q 22 | Page 173

How many atoms are there in 0.25 mole of hydrogen ?

Short Answers | Q 23 | Page 173

Calculate the number of moles in 12.044 × 1025 atoms of phosphorus.

Short Answers | Q 24 | Page 172

Calculate the number of molecules present in a drop of chloroform (CHCl3) weighing 0.0239 g.

(Atomic masses : C = 12 u ; H = 1 u ; Cl = 35.5 u)

Short Answers | Q 25 | Page 172

What is the mass of 5 moles of sodium carbonate (Na2CO3) ?

(Atomic masses : Na = 23 u ; C = 12 u ; O = 16 u)

Short Answers | Q 26 | Page 173

Calculate the number of molecules in 4 g of oxygen.

Short Answers | Q 27 | Page 173

How many moles are represented by 100 g of glucose, C6H12O6 ? (C = 12 u, H = 1 u, O = 16 u)

Short Answers | Q 28 | Page 173

Calculate the mass in grams of 0.17 mole of hydrogen sulphide, H2S. (Atomic masses : H = 1 u, S = 32 u)

Short Answers | Q 29 | Page 173

Show by means of calculations that 5 moles of CO2 and 5 moles of H2O do not have the same mass. How much is the difference in their masses ?

Short Answers | Q 30 | Page 173

Calculate the mole ratio of 240 g of calcium and 240 g of magnesium. (Ca = 40 u ; Mg = 24 u)

Long Answers | Q 31.1 | Page 173

Define mole. What are the two things that a mole represents.

Long Answers | Q 31.2 | Page 173

What weight of each element is present in 1.5 moles of sodium sulphite, Na2SO3 ?

(Atomic masses : Na = 23 u ; S = 32 u ; O = 16 u)

Long Answers | Q 32.1 | Page 173

What is meant by ‘a mole of carbon atoms’ ?

Long Answers | Q 32.2 | Page 173

Which has more atoms, 50 g of aluminium or 50 g of iron ? Illustrate your answer with the help of calculations.

(Atomic masses : Al = 27 u ; Fe = 25 u)

Long Answers | Q 33.1 | Page 173

Define gram atomic mass of a substance. How much is the gram atomic mass of oxygen ?

Long Answers | Q 33.2 | Page 173

How many moles of oxygen atoms are present in one mole of the following compounds ?

  1. Al2O3
  2. CO2
  3. Cl2O7
  4. H2SO4
  5. Al2(SO4)3
Long Answers | Q 34.1 | Page 173

Define gram molecular mass of a substance. How much is the gram molecular mass of oxygen ?

Long Answers | Q 34.2 | Page 173

If sulphur exists as S8 molecules, calculate the number of moles in 100 g of sulphur. (S = 32 u)

Long Answers | Q 35.1 | Page 173

What is meant by the ‘molar mass’ of a substance ? State the unit in which molar mass is usually expressed.

Long Answers | Q 35.2 | Page 173

Calculate the molar masses of the following substances. Write the results with proper units.

  1. Ozone molecule, O3
  2. Ethanoic acid, CH3COOH
Multiple Choice Questions | Q 36 | Page 173

Which of the following pair of elements represents a mole ratio of 1 : 1 ?

  • 10 g of calcium and 12 g of magnesium

  • 12 g of magnesium and 6 g of carbon

  • 12 g of magnesium and 6 g of carbon

  • 20 g of sodium and 20 g of calcium

Multiple Choice Questions | Q 37 | Page 173

Which of the following correctly represents 360 g of water ?

  1. 2 moles of H2O
  2. 20 moles of water
  3. 6.022 × 1023 molecules of water
  4. 1.2044 × 1025 molecules of water
  • (i)

  • (i) and (iv)

  • (ii) and (iii)

  • (ii) and (iv)

Multiple Choice Questions | Q 38 | Page 173

If 32 g of sulphur has x atoms, then the number of atoms in 32 g of oxygen will be :

  • `x/2`

  • 2x

  • x

  • 4x

Multiple Choice Questions | Q 39 | Page 173

A student wants to have 3.011 × 1023 atoms each of magnesium and carbon elements. For this purpose, he will have to weigh :

  • 24 g of magnesium and 6 g of carbon

  • 12 g of carbon and 24 g of magnesium

  • 20 g of magnesium and 10 g of carbon

  • 12 g of magnesium and 6 g of carbon

Multiple Choice Questions | Q 40 | Page 173

The ratio of moles of atoms in 12 g of magnesium and 16 g of sulphur will be :

  • 3 : 4

  • 4 : 3

  • 1 : 1

  • 1 : 2

Multiple Choice Questions | Q 41 | Page 173

If 12 gram of carbon has x atoms, then the number of atoms in 12 grams of magnesium will be :

  • x

  • 2x

  • `x/2`

  • 1.5 x

Hots Questions | Q 42 | Page 174

Which of the following has the maximum number of atoms ?

  • 18 g of H2O

  • 18 g of O2

  •  18 g of CO2

  • 18 g of CH4

Hots Questions | Q 43 | Page 174

If 1 gram of sulphur dioxide contains x molecules, how many molecules will be present in 1 gram of oxygen ?

(S = 32 u ; O = 16 u)

Hots Questions | Q 44 | Page 174

The mass of one molecule of a substance is 4.65 × 10–23 g. What is its molecular mass ? What could this substance be ?

Hots Questions | Q 45 | Page 174

Which contains more molecules, 10 g of sulphur dioxide (SO2) or 10 g of oxygen (O2) ?

(Atomic masses : S = 32 u ; O = 16 u)

Hots Questions | Q 46 | Page 174

What weight of oxygen gas will contain the same number of molecules as 56 g of nitrogen gas ?

(O = 16 u ; N = 14 u)

Hots Questions | Q 47 | Page 174

What mass of nitrogen, N2, will contain the same number of molecules as 1.8 g of water, H2O ?

(Atomic masses : N = 14 u ; H = 1 u ; O = 16 u)

Hots Questions | Q 48 | Page 174

If one gram of sulphur contains x atoms, calculate the number of atoms in one gram of oxygen element.

(Atomic masses : S = 32 u ; O = 16 u)

Hots Questions | Q 49 | Page 174

How many grams of magnesium will have the same number of atoms as 6 grams of carbon ?
(Mg = 24 u ; C = 12 u )

Hots Questions | Q 50 | Page 174

The mass of one atom of an element X is 2.0 × 10–23 g.

Calculate the atomic mass of element X.

What could element X be ?

Chapter 3: Atoms and Molecules

Very Short AnswersShort AnswersLong AnswersMultiple Choice QuestionsOthersHots Questions

Lakhmir Singh Class 9 Chemistry - Science Part 2

Class 9 Chemistry - Science Part 2 - Shaalaa.com

Lakhmir Singh solutions for Class 9 Science chapter 3 - Atoms and Molecules

Lakhmir Singh solutions for Class 9 Science chapter 3 (Atoms and Molecules) include all questions with solution and detail explanation. This will clear students doubts about any question and improve application skills while preparing for board exams. The detailed, step-by-step solutions will help you understand the concepts better and clear your confusions, if any. Shaalaa.com has the CBSE Class 9 Chemistry - Science Part 2 solutions in a manner that help students grasp basic concepts better and faster.

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Concepts covered in Class 9 Science chapter 3 Atoms and Molecules are Atoms and Molecules Numericals, Mole Concept, Formula Unit Mass, Molecular Mass, Writing Chemical Formulae - Formulae of Simple Compounds, Concept of Ion, Molecules of Compounds, Molecules of Elements, Concept of Molecule, Atomic Mass, Modern Day Symbols of Atoms of Different Elements, Concept of Atom, Law of Constant Proportions, Law of Conservation of Mass, Atoms and Molecules Introduction.

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