NCERT Solutions for Class 9 Science Chapter 3 Atoms and Molecules

NCERT Solutions for Class 9 Science Chapter 3 Atoms and Molecules help students understand the fundamental concepts of atoms, molecules, laws of chemical combination, and chemical formulas. This chapter explains how matter is made up of tiny particles and how these particles combine to form compounds.

Prepared according to the latest CBSE Class 9 Science syllabus, these solutions provide clear and structured answers to all textbook questions. Practicing these solutions helps students build strong concepts and perform better in exams.

NCERT Solutions for Class 9 Science Chapter 3 Atoms and Molecules

Atoms and Molecules
Medium
Q.
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.
Atoms and Molecules
Medium
Q.
When 3.0 g of carbon is burnt in 8.00 g oxygen, 11.00g 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 combination will govern your answer?
Atoms and Molecules
Medium
Q.
What are polyatomic ions? Give examples.
Atoms and Molecules
Medium
Q.
Write the chemical formulae of the following.

(a) Magnesium chloride

(b) Calcium oxide

(c) Copper nitrate

(d) Aluminium chloride

(e) Calcium carbonate
Atoms and Molecules
Medium
Q.
Give the names of the elements present in the following compounds.

(a) Quick lime

(b) Hydrogen bromide

(c) Baking powder

(d) Potassium sulphate.
Atoms and Molecules
Medium
Q.
Calculate the molar mass of the following substances.

(a) Ethyne, C2H2

(b) Sulphur molecule, S8

(c) Phosphorus molecule, P4 (Atomic mass of phosphorus = 31)

(d) Hydrochloric acid, HCl

(e) Nitric acid, HNO3
Atoms and Molecules
Medium
Q.
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) ?
Atoms and Molecules
Medium
Q.
Convert into mole:

(a) 12 g of oxygen gas

(b) 20 g of water

(c) 22 g of carbon dioxide
Atoms and Molecules
Medium
Q.
What is the mass of:

(a) 0.2 mole of oxygen atoms?

(b) 0.5 mole of water molecules?
Atoms and Molecules
Medium
Q.
Calculate the number of molecules of sulphur (S8) present in 16 g of solid sulphur.
Atoms and Molecules
Medium
Q.
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)

NCERT Solutions for Class 9 Science Chapter 3 Atoms and Molecules

Q1. What is an atom?

Ans: An atom is the smallest unit of an element that can take part in a chemical reaction.

Q2. What is a molecule?

Ans: A molecule is the smallest particle of a substance that can exist independently and shows all the properties of that substance.

Q3. State the law of conservation of mass.

Ans: Mass can neither be created nor destroyed in a chemical reaction; it remains constant.

Q4. State the law of constant proportions.

Ans: A chemical compound always contains the same elements in the same proportion by mass.

Q5. What is a chemical formula?

Ans: A chemical formula represents the composition of a compound using symbols and numbers.

Q6. What is molar mass?

Ans: Molar mass is the mass of one mole of a substance, expressed in grams per mole (g/mol).

Q7. Define mole.

Ans: A mole is the amount of substance containing 6.022 × 10²³ particles.

Q8. What is the valency of an element?

Ans: Valency is the combining capacity of an element.

FAQs – Class 9 Science Chapter 3 Atoms and Molecules

Q1. What is the importance of the mole concept?
The mole concept helps in measuring large numbers of atoms and molecules easily.

Q2. What is atomic mass?
Atomic mass is the mass of an atom relative to 1/12th mass of carbon-12 atom.

Q3. What is molecular mass?
Molecular mass is the sum of atomic masses of all atoms in a molecule.

Q4. Why are chemical formulas important?
They help in understanding the composition and structure of compounds.

Q5. Why is this chapter important for exams?
This chapter includes numericals and concept-based questions, which are frequently asked in exams.