Chapter – Atoms and Molecules (Class 9 Science)

NCERT Intext + Exercise Questions and Answers

Intext Questions (Page 32–33)

Q1. 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 of water, and 8.2 g of sodium ethanoate. Show that this reaction follows the law of conservation of mass.
Answer:

Total mass of reactants = 5.3 g + 6.0 g = 11.3 g
Total mass of products = 2.2 g + 0.9 g + 8.2 g = 11.3 g
Since, mass of reactants = mass of products, the reaction follows the law of conservation of mass.

Q2. Hydrogen + chlorine → hydrogen chloride. Show this reaction follows the law of conservation of mass.
Answer:

Mass of hydrogen = 1 g
Mass of chlorine = 35.5 g
Total mass of reactants = 36.5 g
Mass of hydrogen chloride formed = 36.5 g
Since mass is conserved, the law is verified.

Intext Questions (Page 35–36)

Q1. Define atomic mass unit.
Answer:
1 atomic mass unit (amu or u) is defined as 1/12th the mass of one atom of carbon-12 isotope.

Q2. Why can’t atoms be seen with naked eyes?
Answer:
Atoms are extremely small in size (~10⁻¹⁰ m), so they cannot be seen with naked eyes. Even a powerful microscope shows them only faintly.

Intext Questions (Page 39–40)

Q1. Write the chemical formula of the following compounds:
(a) Ammonium carbonate
(b) Calcium sulphate
(c) Sodium phosphate
(d) Calcium hydroxide
Answer:
(a) (NH₄)₂CO₃
(b) CaSO₄
(c) Na₃PO₄
(d) Ca(OH)₂

Intext Questions (Page 42–43)

Q1. Write down the names of compounds represented by the following formulae:
(a) Al₂(SO₄)₃
(b) CaCl₂
(c) K₂SO₄
(d) KNO₃
(e) CaCO₃
Answer:
(a) Aluminium sulphate
(b) Calcium chloride
(c) Potassium sulphate
(d) Potassium nitrate
(e) Calcium carbonate

Q2. What is meant by the term chemical formula?
Answer:
A chemical formula is a symbolic representation of the composition of a compound, showing the elements present and their ratio using chemical symbols and subscripts.

Intext Questions (Page 46–47)

Q1. If one mole of carbon atoms weighs 12 g, what is the mass of 1 atom of carbon?(Out of syllabus).
Answer:

1 mole = 6.022 × 10²³ atoms
Mass of 1 mole C = 12 g
Mass of 1 atom = 12 ÷ (6.022 × 10²³) = 1.99 × 10⁻²³ g

Q2. Which has more number of atoms, 100 g of sodium or 100 g of iron?(Out of syllabus)
Answer:

Atomic mass of Na = 23 g/mol → Moles in 100 g = 100/23 = 4.35 mol
Atomic mass of Fe = 56 g/mol → Moles in 100 g = 100/56 = 1.78 mol
Since number of atoms ∝ moles, sodium has more atoms.

NCERT Back Exercise Questions (Page 48–49)

Q1. A 0.24 g sample of a 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.
Answer:

% of B = (0.096 ÷ 0.24) × 100 = 40%
% of O = (0.144 ÷ 0.24) × 100 = 60%

Q2. When 3 g of carbon is burnt in 8 g of oxygen, 11 g of carbon dioxide is produced. What mass of CO₂ will be formed when 3 g of carbon is burnt in 50 g of oxygen? Which law of chemical combination will govern your answer?
Answer:

3 g C + 8 g O₂ → 11 g CO₂
Here O₂ is in excess (50 g), but only 8 g reacts.
So, CO₂ formed = 11 g.
Law followed: Law of definite proportions.

Q3. What are the postulates of Dalton’s atomic theory?
Answer:

Matter consists of indivisible particles called atoms.
Atoms of a given element are identical in mass and properties.
Atoms of different elements differ in mass and properties.
Atoms combine in simple whole-number ratios to form compounds.
Atoms cannot be created or destroyed in a chemical reaction.

Q4. Define atomic mass.
Answer:
Atomic mass is the mass of an atom of an element, expressed in atomic mass units (u), which is equal to 1/12 the mass of one carbon-12 atom.

Q5. What is meant by the molecule of an element? Give examples.
Answer:
A molecule of an element is formed when two or more atoms of the same element combine chemically.
Examples: O₂, H₂, N₂, Cl₂.

Q6. How do atoms exist?
Answer:
Atoms exist:

Independently (noble gases: He, Ne, Ar)
As molecules (H₂, O₂, N₂)
As ions (Na⁺, Cl⁻, Ca²⁺).

Q7. Write down the names of compounds represented by the following formulae:
(a) H₂S
(b) CO₂
(c) CH₄
(d) NH₃
Answer:
(a) Hydrogen sulphide
(b) Carbon dioxide
(c) Methane
(d) Ammonia

Q8. Write the chemical formulae of the following:
(a) Magnesium chloride
(b) Calcium oxide
(c) Copper nitrate
(d) Aluminium chloride
(e) Calcium carbonate
Answer:
(a) MgCl₂
(b) CaO
(c) Cu(NO₃)₂
(d) AlCl₃
(e) CaCO₃

Q9. Calculate the molar mass of the following substances:
(a) Ethyne, C₂H₂
(b) Sulphur molecule, S₈
(c) Phosphorus molecule, P₄
(d) Hydrochloric acid, HCl
(e) Nitric acid, HNO₃
Answer:
(a) C₂H₂ = (2 × 12) + (2 × 1) = 26 g/mol
(b) S₈ = (8 × 32) = 256 g/mol
(c) P₄ = (4 × 31) = 124 g/mol
(d) HCl = 1 + 35.5 = 36.5 g/mol
(e) HNO₃ = 1 + 14 + (3 × 16) = 63 g/mol

kindly comment if any help is required.

Monday, 1 September 2025