Lakhmir Singh solutions for Chemistry [English] Class 9 chapter 3 - Atoms and Molecules [Latest edition]

Write the full form of IUPAC.

Name the scientist who gave law of conservation of mass.

Name the scientist who gave law of constant proportions.

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

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

Name the scientist who gave atomic theory of matter.

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

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

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

Name any two laws of chemical combination.

‘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?

‘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 ?

What are the building blocks of matter ?

How is the size of an atom indicated ?

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

Write the relation between nanometre and metre.

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

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

State whether the following statement is true or false:

The symbol of element cobalt is CO.

Define 'molecular mass' of a substance.

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

Fill in the following blanks with suitable words :

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

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 ....................

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

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

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

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

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

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

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

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

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

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

State the chemical symbols for the following elements :

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

Name the elements represented by the following symbols :

Hg, Pb, Au, Ag, Sn

What is meant by atomicity ? Explain with two examples.

What is the atomicity of the following ?

1. Oxygen
2. Ozone
3. Neon
4. Sulphur
5. Phosphorus
6. Sodium

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

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

Explain the difference between 2N and N2.

What do the following abbreviations stand for ?

1. O
2. 2O
3. O₂
4. 3O₂

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

In what form does oxygen gas occur in nature ?

In what form do noble gases occur in nature?

What is the difference between 2H and H₂ ?

What do the following denote ?

1. N
2. 2N
3. N₂
4. 2N₂

What is the significance of the formula of a substance ?

What is the significance of the formula H₂O ?

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

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)

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)

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)

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)

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)

Calculate the molecular masses of the following compounds  :

Methane, CH₄

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

Calculate the molecular masses of the following compounds  :

Ethane, C₂H₆ 

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

Calculate the molecular masses of the following compounds  :

Ethene, C₂H₄ 

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

Calculate the molecular masses of the following compounds  :

Ethyne, C₂H₂

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

Calculate the molecular masses of the following compounds :

Methanol, CH3OH

Calculate the molecular masses of the following compounds :

Ethanol, C₂H₅OH

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

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

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

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

Calculate the molecular masses of the following compounds :

Hydrogen sulphide, H₂S

Calculate the molecular masses of the following compounds :

Carbon disulphide, CS₂

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

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

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

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

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

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

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

Explain the significance of the symbol H.

What is an atom ? How do atoms usually exist ?

What is a molecule ? Explain with an example.

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

Define atomic mass unit. What is its symbol ?

Define atomic mass of an element.

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

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

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

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

The atomic theory of matter was proposed by :

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

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

The law of conservation of mass was given by :

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

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 :

The law of constant proportions was given by :

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

One nm is equal to :

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 :

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

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

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

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

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

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.

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 ?

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.

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 ?

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 ?

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.

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 ?

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

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

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

Define ‘formula mass’ of a compound.

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

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

State whether the following statements are true or false:

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

State whether the following statements are true or false :

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

Write down the formulae for the following compounds :

Calcium oxide

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

Write down the formulae for the following compounds :

Magnesium hydroxide

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

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

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

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..............

Fill in the following blank with suitable word:

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

Fill in the following blank with suitable word :

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

Fill in the following blank with suitable word:

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

Fill in the following blank with suitable word:

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

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

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

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

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 ?

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 ?

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

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

Work out the formula for magnesium hydrogencarbonate.

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

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

Work out the formulae for the following compounds :

Sodium oxide

Work out the formulae for the following compounds:

Calcium carbonate

Calculate the formula masses of the following compounds :

Sodium oxide, Na₂O

Calculate the formula masses of the following compounds :

Calcium carbonate

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

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

1. CH₃COONa 
2. NaCl
3. H₂
4. NH₄NO3

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

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

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

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

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

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

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

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 ?

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

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

1. chloride
2. oxide
3. sulphate
4. carbonate
5. nitrate

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

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)

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

Which of the following represents a correct chemical formula ?

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

The anion of an element has ______.

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

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

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

The cation of an element has :

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 :

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

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

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

Molecular compounds are usually formed by the combination between :

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

An element A forms an oxide A2O5.

What is the valency of element A ?

An element A forms an oxide A₂O₅.

What will be the formula of chloride of A ?

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.

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 ?

The formula of carbonate of a metal M is M₂CO₃.

What will be the formula of its iodide ?   

The formula of carbonate of a metal M is M₂CO₃.

What will be the formula of its nitride ?

The formula of carbonate of a metal M is M₂CO₃.

 What will be the formula of its phosphate ?

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 ?

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

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

What is the numerical value of Avogadro number ?

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

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

What name is given to the number 6.022 × 1023 ?

Convert 12 g of oxygen gas into moles.

How many moles are 3.6 g of water ?

What is the mass of 0.2 mole of oxygen atoms ?

Find the mass of 2 moles of nitrogen atoms.

Fill in the following blanks :

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

Fill in the following blanks :

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

Fill in the following blanks :

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

Fill in the following blanks :

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

Fill in the following blanks :

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

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

What name is given to this number ?

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

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

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

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

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

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

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

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

Calculate the mass of 3.011 × 1024 atoms of carbon.

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

How many atoms are there in 0.25 mole of hydrogen ?

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

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)

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

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

Calculate the number of molecules in 4 g of oxygen.

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

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

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 ?

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

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

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)

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

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)

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

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

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

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

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

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

1. Ozone molecule, O3
2. Ethanoic acid, CH3COOH

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

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

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

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

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

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

Which of the following has the maximum number of atoms ?

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)

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

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

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

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

(O = 16 u ; N = 14 u)

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)

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)

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

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 ?