---

NCERT Solutions for Class 9 Science Chapter 3- Atoms and Molecules

This chapter holds a weightage of 23 marks in the final examinations. The questions are not specific and can be asked from any topic. The students are therefore advised to be thorough with the entire chapter. The chemical formulae and the numerical should be practised well.

The important topics provided in this chapter include:

3.1 Laws of Chemical Combination

3.2 What is an Atom?

3.3 What is a Molecule?

3.4 Writing Chemical Formulae

3.5 Molecular Mass and Mole Concept

Frequently Asked Questions on Chapter 3 Atoms And Molecules

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

The postulate of Dalton’s Atomic theory which is a result of the law of conservation of mass is,
“Atoms can neither be created nor destroyed”.

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

The postulate of Dalton’s atomic theory that can explain the law of definite proportions is – the
relative number and kinds of atoms are equal in given compounds.

Define the atomic mass unit?

An atomic mass unit is a unit of mass used to express weights of atoms and molecules where one
atomic mass is equal to 1/12th the mass of one carbon-12 atom.

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

Firstly, atoms are miniscule in nature, measured in nanometers. Secondly, except for atoms of noble
gasses, they do not exist independently. Hence, an atom cannot be visible to the naked eyes.

What is meant by the term chemical formula?

Chemical formula is the symbolic representation of a chemical compound. For example: The chemical formula of hydrochloric acid is HCl.

What are polyatomic ions? Give examples.

Polyatomic ions are ions that contain more than one atom but they behave as a single unit

Example: CO₃^2- , H₂PO₄^–

What is the mass of -(a) 1 mole of nitrogen atoms?(b) 4 moles of aluminium atoms((Atomic mass of aluminium =27)?(c) 10 moles of sodium sulphite (Na₂SO₃)?

The mass of the above mentioned list is as follows:

(a) Atomic mass of nitrogen atoms = 14u

Mass of 1 mole of nitrogen atoms= Atomic mass of nitrogen atoms

Therefore, mass of 1 mole of nitrogen atom is 14g

(b) Atomic mass of aluminium =27u

Mass of 1 mole of aluminium atoms = 27g

1 mole of aluminium atoms = 27g, 4 moles of aluminium atoms = 4 x 27 = 108g

(c) Mass of 1 mole of sodium sulphite Na₂SO₃ = Molecular mass of sodium sulphite = 2 x Mass of Na + Mass of S + 3 x Mass of O = (2 x 23) + 32 +(3x 16) = 46+32+48 = 126g

Therefore, mass of 10 moles of Na₂SO₃ = 10 x 126 = 1260g