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NCERT solutions Science Class 9 chapter 3 Atoms and Molecules

Chapter 3 - Atoms and Molecules

Page 33

In a reaction, 5.3 g of sodium carbonate reacted with 6 g of ethanoic acid. The products were 2.2 g of carbon dioxide, 0.9 g water and 8.2 g of sodium ethanoate. Show that these observations are in agreement with the law of conservation of mass.

Sodium carbonate + ethanoic acid → sodium ethanoate + carbon dioxide + water

Q 1 | Page 33

Hydrogen and oxygen combine in the ratio of 1:8 by mass to form water. What mass of oxygen gas would be required to react completely with 3 g of hydrogen gas?

Q 2 | Page 33

Which postulate of Dalton’s atomic theory is the result of the law of conservation of mass?

Q 3 | Page 33

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

Q 4 | Page 33

Page 35

Define atomic mass unit.

Q 1 | Page 35

Why is it not possible to see an atom with naked eyes?

Q 2 | Page 35

Page 39

Write down the formulae of sodium oxide.

Q 1.1 | Page 39

Write down the formulae of aluminium chloride.

Q 1.2 | Page 39

Write down the formulae of sodium sulphide.

Q 1.3 | Page 39

Write down the formulae of magnesium hydroxide.

Q 1.4 | Page 39

Write down the name of compound represented by the following formulae:- Al2(SO4)3

Q 2.1 | Page 39

Write down the name of compound represented by the following formulae:-

K2SO4

Q 2.2 | Page 39

Write down the name of compound represented by the following formulae:- CaCl2

Q 2.3 | Page 39

Write down the name of compound represented by the following formulae:-KNO3

Q 2.4 | Page 39

Write down the name of compound represented by the following formulae:- CaCO3

Q 2.5 | Page 39

What is meant by the term chemical formula?

Q 3 | Page 39

How many atoms are present in a H2S molecule?

Q 4.1 | Page 39

How many atoms are present in a PO43− ion?

Q 4.2 | Page 39

Page 40

Calculate the molecular masses of H2.

Q 1.1 | Page 40

Calculate the molecular masses of O2.

Q 1.2 | Page 40

Calculate the molecular masses of Cl2.

Q 1.3 | Page 40

Calculate the molecular masses of CO2.

Q 1.4 | Page 40

Calculate the molecular masses of CH4.

Q 1.5 | Page 40

Calculate the molecular masses of C2H6.

Q 1.6 | Page 40

Calculate the molecular masses of C2H4.

Q 1.7 | Page 40

Calculate the molecular masses of NH3.

Q 1.8 | Page 40

Calculate the molecular masses of CH3OH.

Q 1.9 | Page 40

Calculate the formula unit masses of ZnO, given atomic masses of Zn = 65 u and O = 16 u.

Q 2.1 | Page 40

Calculate the formula unit masses of Na2O, given atomic masses of Na = 23 u and O = 16 u.

Q 2.2 | Page 40

Calculate the formula unit masses of K2CO3, given atomic masses of K = 39 u, C = 12 u, and O = 16 u.

Q 2.3 | Page 40

Pages 42 - 44

If one mole of carbon atoms weighs 12 gram, what is the mass (in gram) of 1 atom of carbon?

Q 1 | Page 42

A 0.24 g sample of compound of oxygen and boron was found by analysis to contain 0.096 g of boron and 0.144 g of oxygen. Calculate the percentage composition of the compound by weight.

Q 1 | Page 43

Which has more number of atoms, 100 grams of sodium or 100 grams of iron (given, atomic mass of Na = 23 u, Fe = 56 u)?

Q 2 | Page 42

When 3.0 g of carbon is burnt in 8.00 g oxygen, 11.00 g of carbon dioxide is produced. What mass of carbon dioxide will be formed when 3.00 g of carbon is burnt in 50.00 g of oxygen? Which law of chemical combinations will govern your answer?

Q 2 | Page 43

What are polyatomic ions? Give examples?

Q 3 | Page 44

Write the chemical formulae of the Magnesium chloride.

Q 4.1 | Page 44

Write the chemical formulae of the Calcium oxide.

Q 4.2 | Page 44

Write the chemical formulae of the Copper nitrate.

Q 4.3 | Page 44

Write the chemical formulae of the Calcium carbonate.

Q 4.5 | Page 44

Give the names of the elements present in the following compound: Quick lime

Q 5.1 | Page 44

Give the names of the elements present in the following compound: Hydrogen bromide

Q 5.2 | Page 44

Give the names of the elements present in the following compound: Baking powder

Q 5.3 | Page 44

Give the names of the elements present in the following compound: Potassium sulphate

Q 5.4 | Page 44

Calculate the molar mass of the following substance: Ethyne, C2H2

Q 6.1 | Page 44

Calculate the molar mass of the following substance

Phosphorus molecule, P4 (atomic mass of phosphorus = 31)

Q 6.2 | Page 44

Calculate the molar mass of the following substance: Sulphur molecule, S8

Q 6.3 | Page 44

Calculate the molar mass of the following substance: Hydrochloric acid, HCl

Q 6.4 | Page 44

Calculate the molar mass of the following substance: Nitric acid, HNO3

Q 6.5 | Page 44

What is the mass of 1 mole of nitrogen atoms?

Q 7.1 | Page 44

What is the mass of 4 moles of aluminium atoms (Atomic mass of aluminium = 27)?

Q 7.2 | Page 44

What is the mass of 10 moles of sodium sulphite (Na2SO3)?

Q 7.3 | Page 44

Convert into mole.

12 g of oxygen gas

Q 8.1 | Page 44

Convert into mole.

20 g of water

Q 8.2 | Page 44

Convert into mole.

22 g of carbon dioxide

Q 8.3 | Page 44

What is the mass of 0.2 mole of oxygen atoms?

Q 9.1 | Page 44

What is the mass of 0.5 mole of water molecules?

Q 9.2 | Page 44

Calculate the number of molecules of sulphur (S8) present in 16 g of solid sulphur.

Q 10 | Page 44

Calculate the number of aluminium ions present in 0.051 g of aluminium oxide.

(Hint: The mass of an ion is the same as that of an atom of the same element. Atomic mass of Al = 27 u)

Q 11 | Page 44

Extra questions

The average atomic mass of a sample of an element X is 16.2 u. What are the percentages of isotopes ""_8^16X" and """_8^18X  in the sample?

If bromine atom is available in the form of, say, two isotopes ""_35^79Br (49.7%)" and """_35^81Br (50.3%), calculate the average atomic mass of bromine atom.

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