🧪 Atoms and Molecules – MCQs for Class 9 (NCERT)

Section 3: Writing Chemical Formulae and Equations (Q51–Q75)

Q51. The correct formula of aluminium oxide is:

A. AlO
B. AlO₂
C. Al₂O₃
D. Al₃O₂

✅ Correct Answer: C. Al₂O₃

Explanation:

• A. AlO → Wrong, charges don’t balance.
• B. AlO₂ → Wrong, Al³⁺ and O²⁻ don’t balance.
• C. Al₂O₃ → Correct, Al³⁺ × 2 = +6, O²⁻ × 3 = –6. Balanced.
• D. Al₃O₂ → Wrong, imbalance in charges.

Q52. The formula of ammonium carbonate is:

A. NH₄CO₃
B. (NH₄)₂CO₃
C. (NH₄)₃CO₃
D. NH₄(CO₃)₂

✅ Correct Answer: B. (NH₄)₂CO₃

Explanation:

• A. NH₄CO₃ → Wrong, missing one NH₄⁺.
• B. (NH₄)₂CO₃ → Correct, 2 × +1 balances CO₃²⁻.
• C. (NH₄)₃CO₃ → Wrong, charge mismatch.
• D. NH₄(CO₃)₂ → Wrong formula.

Q53. The valency of sulphur in H₂SO₄ is:

A. 2
B. 4
C. 6
D. 8

✅ Correct Answer: C. 6

Explanation:

• A. 2 → Wrong, sulphur is not divalent here.
• B. 4 → Wrong, valency is higher.
• C. 6 → Correct, sulphur shows +6 oxidation state in H₂SO₄.
• D. 8 → Wrong, no +8 valency.

Q54. The correct chemical formula of calcium hydroxide is:

A. CaOH
B. Ca(OH)₂
C. Ca₂OH
D. Ca₂(OH)₂

✅ Correct Answer: B. Ca(OH)₂

Explanation:

• A. CaOH → Wrong, only one OH⁻.
• B. Ca(OH)₂ → Correct, Ca²⁺ needs 2 OH⁻ to balance.
• C. Ca₂OH → Wrong formula.
• D. Ca₂(OH)₂ → Wrong, would give +4, –2.

Q55. Which of the following is a correct balanced chemical equation?

A. H₂ + O₂ → H₂O
B. 2H₂ + O₂ → 2H₂O
C. 2H₂ + 2O₂ → 2H₂O
D. H₂ + 2O₂ → 2H₂O

✅ Correct Answer: B. 2H₂ + O₂ → 2H₂O

Explanation:

• A. H₂ + O₂ → H₂O → Wrong, not balanced.
• B. Correct, atoms balanced.
• C. Wrong, oxygen not balanced.
• D. Wrong, O₂ count incorrect.

Q56. The symbol (aq) in a chemical equation denotes:

A. Solid
B. Liquid
C. Aqueous solution
D. Gas

✅ Correct Answer: C. Aqueous solution

Explanation:

• A. Solid → (s).
• B. Liquid → (l).
• C. Aqueous → Correct, dissolved in water.
• D. Gas → (g).

Q57. The formula of magnesium chloride is:

A. MgCl
B. MgCl₂
C. Mg₂Cl
D. Mg₂Cl₃

✅ Correct Answer: B. MgCl₂

Explanation:

• A. MgCl → Wrong, charges not balanced.
• B. MgCl₂ → Correct, Mg²⁺ balanced by 2Cl⁻.
• C. Mg₂Cl → Wrong.
• D. Mg₂Cl₃ → Wrong.

Q58. The chemical reaction 2KClO₃ → 2KCl + 3O₂ is an example of:

A. Combination reaction
B. Decomposition reaction
C. Double displacement reaction
D. Neutralisation reaction

✅ Correct Answer: B. Decomposition reaction

Explanation:

• A. Combination → Wrong, 2 reactants form 1 product.
• B. Decomposition → Correct, 1 reactant breaks into 2 products.
• C. Double displacement → Wrong, exchange doesn’t occur.
• D. Neutralisation → Wrong, acid + base.

Q59. The valency of aluminium is:

A. 1
B. 2
C. 3
D. 4

✅ Correct Answer: C. 3

Explanation:

• A. 1 → Wrong.
• B. 2 → Wrong.
• C. 3 → Correct, Al³⁺ ion.
• D. 4 → Wrong.

Q60. Which of the following is a skeletal chemical equation?

A. 2H₂ + O₂ → 2H₂O
B. H₂ + O₂ → H₂O
C. NaOH + HCl → NaCl + H₂O
D. Zn + H₂SO₄ → ZnSO₄ + H₂

✅ Correct Answer: B. H₂ + O₂ → H₂O

Explanation:

• A. Balanced equation.
• B. Correct, skeletal = unbalanced form.
• C. Balanced.
• D. Balanced.

Q61. The correct formula of sodium sulphate is:

A. NaSO₄
B. Na₂SO₄
C. Na(SO₄)₂
D. Na₂(SO₄)₂

✅ Correct Answer: B. Na₂SO₄

Explanation:

• A. NaSO₄ → Wrong, imbalance.
• B. Correct, Na⁺ × 2 = +2, SO₄²⁻ = –2.
• C. Wrong.
• D. Wrong.

Q62. Which of the following reactions is a neutralisation reaction?

A. HCl + NaOH → NaCl + H₂O
B. 2H₂ + O₂ → 2H₂O
C. CaCO₃ → CaO + CO₂
D. Zn + H₂SO₄ → ZnSO₄ + H₂

✅ Correct Answer: A. HCl + NaOH → NaCl + H₂O

Explanation:

• A. Correct, acid + base → salt + water.
• B. Wrong, combination.
• C. Wrong, decomposition.
• D. Wrong, displacement.

Q63. Which of the following is an example of a double displacement reaction?

A. NaCl + AgNO₃ → NaNO₃ + AgCl
B. Zn + CuSO₄ → ZnSO₄ + Cu
C. 2H₂ + O₂ → 2H₂O
D. CaCO₃ → CaO + CO₂

✅ Correct Answer: A. NaCl + AgNO₃ → NaNO₃ + AgCl

Explanation:

• A. Correct, ions exchange places.
• **B. Wrong, single displacement.
• **C. Wrong, combination.
• **D. Wrong, decomposition.

Q64. The balanced equation for rusting of iron is:

A. Fe + O₂ → Fe₂O₃
B. 4Fe + 3O₂ → 2Fe₂O₃
C. Fe + O₂ + H₂O → Fe(OH)₃
D. 4Fe + 6H₂O + 3O₂ → 4Fe(OH)₃

✅ Correct Answer: D. 4Fe + 6H₂O + 3O₂ → 4Fe(OH)₃

Explanation:

• A. Simplified form, not complete.
• B. Correct for dry oxidation, not rusting.
• C. Incomplete.
• D. Correct, rusting requires oxygen + moisture.

Q65. The symbol (g) in equations means:

A. Gas
B. Ground state
C. Group
D. Gram

✅ Correct Answer: A. Gas

Explanation:

• A. Correct, (g) → gas.
• **B. Wrong, not chemistry notation.
• **C. Wrong.
• **D. Wrong.

Q66. The compound formed between Ca²⁺ and PO₄³⁻ ions has formula:

A. CaPO₄
B. Ca₂PO₄
C. Ca₃(PO₄)₂
D. Ca(PO₄)₃

✅ Correct Answer: C. Ca₃(PO₄)₂

Explanation:

• A. CaPO₄ → Wrong, unbalanced charges.
• B. Ca₂PO₄ → Wrong.
• C. Correct, +6 and –6 balance.
• **D. Wrong.

Q67. The formula of sodium carbonate is:

A. NaCO₃
B. Na₂CO₃
C. Na₂(CO₃)₂
D. Na(CO₃)₂

✅ Correct Answer: B. Na₂CO₃

Explanation:

• **A. Wrong.
• **B. Correct, Na⁺ × 2 balances CO₃²⁻.
• **C. Wrong.
• **D. Wrong.

Q68. Which of the following is NOT a physical state symbol used in equations?

A. (aq)
B. (s)
C. (x)
D. (g)

✅ Correct Answer: C. (x)

Explanation:

• A. (aq) → aqueous.
• B. (s) → solid.
• C. (x) → Not used.
• D. (g) → gas.

Q69. The chemical equation for photosynthesis is:

A. CO₂ + H₂O → C₆H₁₂O₆ + O₂
B. 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
C. C + O₂ → CO₂
D. H₂ + Cl₂ → 2HCl

✅ Correct Answer: B. 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

Explanation:

• **A. Wrong, not balanced.
• **B. Correct, balanced photosynthesis.
• **C. Wrong, combustion.
• **D. Wrong, combination.

Q70. Which of the following reactions is endothermic?

A. 2H₂ + O₂ → 2H₂O
B. C + O₂ → CO₂
C. CaCO₃ → CaO + CO₂
D. CH₄ + 2O₂ → CO₂ + 2H₂O

✅ Correct Answer: C. CaCO₃ → CaO + CO₂

Explanation:

• **A. Combustion, exothermic.
• **B. Combustion, exothermic.
• **C. Correct, decomposition requires heat.
• **D. Combustion, exothermic.

Q71. Which of the following is the correct formula of ferrous sulphate?

A. FeSO₄
B. Fe₂(SO₄)₃
C. Fe₂SO₄
D. Fe(SO₄)₂

✅ Correct Answer: A. FeSO₄

Explanation:

• **A. Correct, Fe²⁺ + SO₄²⁻.
• **B. Wrong, ferric sulphate.
• **C. Wrong formula.
• **D. Wrong formula.

Q72. Which is a double displacement reaction?

A. AgNO₃ + NaCl → AgCl + NaNO₃
B. Zn + HCl → ZnCl₂ + H₂
C. 2Mg + O₂ → 2MgO
D. CaCO₃ → CaO + CO₂

✅ Correct Answer: A. AgNO₃ + NaCl → AgCl + NaNO₃

Explanation:

• **A. Correct, ions exchange.
• **B. Single displacement.
• **C. Combination.
• **D. Decomposition.

Q73. The chemical formula of copper (II) sulphate is:

A. CuSO₄
B. Cu₂SO₄
C. Cu(SO₄)₂
D. Cu₂(SO₄)₃

✅ Correct Answer: A. CuSO₄

Explanation:

• **A. Correct, Cu²⁺ + SO₄²⁻.
• **B. Wrong, charges don’t balance.
• **C. Wrong.
• **D. Wrong.

Q74. Which of the following reactions is exothermic?

A. Respiration (C₆H₁₂O₆ + O₂ → CO₂ + H₂O)
B. Photosynthesis
C. Electrolysis of water
D. CaCO₃ → CaO + CO₂

✅ Correct Answer: A. Respiration

Explanation:

• **A. Respiration releases energy → Correct.
• **B. Photosynthesis needs energy → Endothermic.
• **C. Electrolysis requires electricity → Endothermic.
• **D. Decomposition requires heat → Endothermic.

Q75. The correct formula of zinc nitrate is:

A. ZnNO₃
B. Zn(NO₃)₂
C. Zn₂(NO₃)₃
D. Zn₂NO₃

✅ Correct Answer: B. Zn(NO₃)₂

Explanation:

• **A. Wrong, charges don’t balance.
• **B. Correct, Zn²⁺ balanced by 2 NO₃⁻.
• **C. Wrong.
• **D. Wrong.

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