Class 9 Chemistry Chapter 3: Atoms and Molecules MCQ Quiz with Detailed Answers

The revised CBSE Board exam pattern (2024–25 onwards) has shifted heavily towards objective questions. In Class 9 Science (Chemistry), Term 1 and Term 2 papers now allocate 20–25% weightage to MCQs. Chapter 3 (Atoms and Molecules) is a prime candidate because it tests both conceptual understanding and numerical problem-solving. MCQs are efficient: they force precision (no vague answers), cover wider content in less time, and reduce marking ambiguity. Moreover, aspirants targeting JEE/NEET in Class 11 build speed and accuracy through regular MCQ practice now. The 'Laws of Chemical Combination' and 'mole concept' questions often twist wording to test deeper understanding—not just formula recall. By practising varied MCQ formats (single-answer, assertion-reason, numerical-based), you train your brain to spot trick options and avoid common misconceptions about atomic mass units (u), molecular mass, and avogadro's number (6.022 × 10²³).

**Q1.** Which of the following represents the Law of Conservation of Mass? (A) Matter can be created but not destroyed (B) Mass is neither created nor destroyed in a chemical reaction (C) Atoms are indivisible units (D) Molecules must contain at least two atoms **Answer: (B)** — Lavoisier's law: total mass of reactants = total mass of products. **Q2.** An atom of an element has a mass of 40 u. What is its atomic mass? (A) 40 g (B) 40 u (C) 40 g/mol (D) 40 atoms **Answer: (B)** — Atomic mass is expressed in atomic mass units (u); 1 u ≈ 1.66 × 10⁻²⁷ kg. **Q3.** Which statement defines a molecule? (A) A group of atoms bonded together (B) The smallest unit of an element (C) An atom that has lost electrons (D) A particle with only protons **Answer: (A)** — Molecules are the smallest particles of a compound formed by covalent bonding. **Q4.** The molecular mass of H₂O is approximately: (A) 16 u (B) 18 u (C) 20 u (D) 8 u **Answer: (B)** — H = 1 u, O = 16 u; so H₂O = 1×2 + 16 = 18 u. **Q5.** Which of the following is a diatomic molecule? (A) O₃ (B) H₂ (C) P₄ (D) S₈ **Answer: (B)** — Diatomic means two atoms; H₂, N₂, O₂, Cl₂ are common examples. **Q6.** The atomic mass unit (u) is defined relative to: (A) Hydrogen atom (B) Carbon-12 isotope (C) Oxygen atom (D) Electron mass **Answer: (B)** — One carbon-12 atom = 12 u exactly (NCERT, IUPAC standard). **Q7.** Which law states that in a chemical compound, elements are always present in a fixed ratio by mass? (A) Law of Conservation of Mass (B) Law of Definite Proportions (C) Law of Multiple Proportions (D) Avogadro's Law **Answer: (B)** — Also called Law of Constant Composition; e.g., water always contains H and O in 1:8 mass ratio. **Q8.** Calculate the number of moles in 32 g of oxygen gas (O₂). [Molar mass O₂ = 32 g/mol] (A) 0.5 mol (B) 1 mol (C) 2 mol (D) 4 mol **Answer: (B)** — Moles = mass ÷ molar mass = 32 ÷ 32 = 1 mol. **Q9.** The formula mass of NH₄Cl is: (A) 17 u (B) 35.5 u (C) 53.5 u (D) 71 u **Answer: (C)** — N(14) + H₄(4) + Cl(35.5) = 53.5 u. **Q10.** Avogadro's number is: (A) 6.02 × 10²² (B) 6.02 × 10²³ (C) 6.02 × 10²⁴ (D) 6.02 × 10²⁵ **Answer: (B)** — One mole of any substance contains exactly 6.022 × 10²³ particles (atoms, molecules, or ions).

**Q11.** Which compound has the highest molecular mass? (A) CO₂ (44 u) (B) NH₃ (17 u) (C) H₂SO₄ (98 u) (D) C₂H₆ (30 u) **Answer: (C)** — H₂SO₄ = 2(1) + 32 + 4(16) = 98 u; compare carefully. **Q12.** How many moles of carbon atoms are present in 2 moles of C₂H₅OH (ethanol)? (A) 2 mol (B) 3 mol (C) 4 mol (D) 6 mol **Answer: (C)** — Each C₂H₅OH has 2 C atoms; 2 moles × 2 = 4 mol C atoms. **Q13.** The Law of Multiple Proportions states that when two elements form two different compounds, the ratio of masses of one element (combining with a fixed mass of the other) is in: (A) Simple whole number ratio (B) Irrational ratio (C) Decimal ratio (D) Fractional ratio **Answer: (A)** — Example: CO and CO₂; mass ratio of O = 1:2 (simple whole number). **Q14.** Calculate molar mass of Ca(OH)₂. [Ca = 40, O = 16, H = 1] (A) 57 g/mol (B) 74 g/mol (C) 48 g/mol (D) 41 g/mol **Answer: (B)** — Ca + 2(O + H) = 40 + 2(16 + 1) = 40 + 34 = 74 g/mol. **Q15.** How many grams of H₂ (molar mass = 2 g/mol) are needed to make 3 moles? (A) 2 g (B) 3 g (C) 6 g (D) 9 g **Answer: (C)** — Mass = moles × molar mass = 3 × 2 = 6 g. **Q16.** The valency of nitrogen in NH₃ is: (A) +3 (B) –3 (C) +1 (D) –1 **Answer: (B)** — N shares 3 electrons with H atoms; valency = 3 (or –3 in context). **Q17.** Which set of chemical formulae is correct? (A) Na₂O (sodium oxide), MgCl₂ (magnesium chloride) (B) NaO (sodium oxide), MgCl (magnesium chloride) (C) Na₃O (sodium oxide), Mg₂Cl (magnesium chloride) (D) NaO₂ (sodium oxide), Mg₃Cl₂ (magnesium chloride) **Answer: (A)** — Na⁺ (valency 1), O²⁻ → Na₂O; Mg²⁺, Cl⁻ → MgCl₂. **Q18.** Calculate the number of atoms in 2 moles of Fe. [Avogadro's number = 6.022 × 10²³] (A) 6.022 × 10²³ (B) 1.204 × 10²⁴ (C) 3.011 × 10²³ (D) 2.411 × 10²⁴ **Answer: (B)** — Atoms = moles × Avogadro's number = 2 × 6.022 × 10²³ = 1.204 × 10²⁴. **Q19.** In the compound CaCO₃, the ratio of Ca : C : O by atoms is: (A) 1:1:2 (B) 1:1:3 (C) 2:1:3 (D) 1:2:3 **Answer: (B)** — Formula CaCO₃ has 1 Ca atom, 1 C atom, and 3 O atoms. **Q20.** A sample contains 3.011 × 10²³ molecules of O₂. How many moles of O₂ are present? (A) 0.5 mol (B) 1 mol (C) 2 mol (D) 3 mol **Answer: (A)** — Moles = number of molecules ÷ Avogadro's number = 3.011 × 10²³ ÷ 6.022 × 10²³ = 0.5 mol.

**Q21. Assertion (A):** Atomic mass and atomic mass number are the same. **Reason (R):** Atomic mass number is the sum of protons and neutrons. (A) Both A and R are true, and R is the correct explanation of A (B) Both A and R are true, but R is not the correct explanation of A (C) A is true, but R is false (D) Both A and R are false **Answer: (D)** — Atomic mass (weighted average, includes isotopes) ≠ mass number (sum of p + n); they differ. **Q22. Assertion (A):** One mole of CO₂ has the same number of molecules as one mole of H₂O. **Reason (R):** Both are diatomic molecules. (A) Both A and R are true, and R is the correct explanation of A (B) Both A and R are true, but R is not the correct explanation of A (C) A is true, but R is false (D) A is false, but R is true **Answer: (B)** — A is true (same avogadro's number per mole), but R is false (CO₂ is triatomic, H₂O is triatomic—neither diatomic). **Q23.** In a compound, hydrogen is always: (A) Monovalent (B) Bivalent (C) Trivalent (D) Can have variable valency **Answer: (A)** — Hydrogen shows valency +1 (or –1 in hydrides) consistently; it never forms more than one covalent bond. **Q24. Assertion (A):** The empirical formula of glucose is CH₂O. **Reason (R):** The molecular formula of glucose is C₆H₁₂O₆. (A) Both A and R are true, and R is the correct explanation of A (B) Both A and R are true, but R is not the correct explanation of A (C) A is true, but R is false (D) A is false, but R is true **Answer: (B)** — Both statements are true; C₆H₁₂O₆ simplifies to CH₂O, so A is supported but not directly explained by R (they're related, not explanatory). **Q25.** If 4 g of a gas X occupies 2.24 L at STP (1 mole of gas = 22.4 L at STP), the molar mass of X is: (A) 80 g/mol (B) 40 g/mol (C) 20 g/mol (D) 10 g/mol **Answer: (A)** — Moles = 2.24 ÷ 22.4 = 0.1 mol; Molar mass = mass ÷ moles = 4 ÷ 0.1 = 40 g/mol. [Note: Recalculate—correct answer is (B) 40 g/mol.] **Q26. Assertion (A):** Water has a molar mass of 18 g/mol. **Reason (R):** Water contains two hydrogen atoms and one oxygen atom. (A) Both A and R are true, and R is the correct explanation of A (B) Both A and R are true, but R is not the correct explanation of A (C) A is true, but R is false (D) Both A and R are false **Answer: (A)** — A is true; R explains it: 2(1) + 16 = 18 g/mol. **Q27.** The mass of 0.5 moles of CaCO₃ is (Molar mass CaCO₃ = 100 g/mol): (A) 25 g (B) 50 g (C) 100 g (D) 200 g **Answer: (B)** — Mass = moles × molar mass = 0.5 × 100 = 50 g. **Q28. Assertion (A):** In the compound MgO, the ratio of Mg to O by mass is approximately 3:2. **Reason (R):** Mg has atomic mass 24 u and O has atomic mass 16 u. (A) Both A and R are true, and R is the correct explanation of A (B) Both A and R are true, but R is not the correct explanation of A (C) A is true, but R is false (D) Both A and R are false **Answer: (A)** — In MgO, mass ratio = 24:16 = 3:2; R directly explains A. **Q29.** How many molecules of N₂ are present in 1.4 g of N₂? (Molar mass N₂ = 28 g/mol; Avogadro's number = 6.022 × 10²³) (A) 6.022 × 10²² (B) 3.011 × 10²² (C) 6.022 × 10²³ (D) 1.204 × 10²⁴ **Answer: (B)** — Moles = 1.4 ÷ 28 = 0.05 mol; Molecules = 0.05 × 6.022 × 10²³ = 3.011 × 10²². **Q30. Assertion (A):** The molecular mass of H₂SO₄ is 98 u. **Reason (R):** H₂SO₄ contains 2 H atoms, 1 S atom, and 4 O atoms. (A) Both A and R are true, and R is the correct explanation of A (B) Both A and R are true, but R is not the correct explanation of A (C) A is true, but R is false (D) Both A and R are false **Answer: (A)** — 2(1) + 32 + 4(16) = 98 u; R explains A completely.

**Trap 1: Confusing atomic mass with mass number.** Students often mix up atomic mass (a decimal, e.g., 12.01 u for carbon) with the mass number (a whole number, e.g., 12 for carbon-12). Atomic mass is a weighted average across all stable isotopes; mass number = protons + neutrons in a specific nucleus. Always read the question carefully. **Trap 2: Forgetting subscripts in formulae.** In Q4, many students see H₂O and calculate 1 + 16 = 17 u instead of 2(1) + 16 = 18 u. Always count every atom, including the invisible '1' subscript. **Trap 3: Mixing molar mass (g/mol) with molecular mass (u).** Numerically, they're the same (e.g., H₂O = 18 u = 18 g/mol), but units matter. Molecular mass is in u; molar mass is in g/mol. A careless option might swap units. **Trap 4: Misapplying Avogadro's number.** Many students divide when they should multiply, or vice versa. Memorize: **Moles = particles ÷ 6.022 × 10²³** and **Particles = moles × 6.022 × 10²³**. Also, 'number of molecules' applies to molecular substances (O₂, CO₂); for atoms (Fe, Ca), use 'number of atoms'. **Trap 5: Ignoring the Law of Multiple Proportions.** This law is often tested with CO and CO₂: students must recognize that the mass of O in CO₂ is twice that in CO (when C is constant). Missing this nuance leads to wrong answers in Q13-like problems. **Trap 6: Writing incorrect chemical formulae.** Valency errors are common. Calcium (Ca²⁺) + chloride (Cl⁻) = CaCl₂, not CaCl or Ca₂Cl. Learn the valency table: alkali metals (+1), alkaline earth metals (+2), Al (+3), N (–3), O (–2), Cl (–1). **Trap 7: Confusing empirical and molecular formulae.** Glucose's molecular formula is C₆H₁₂O₆, but its empirical formula is CH₂O (the simplest whole-number ratio). Some options offer one when the other is required. **Trap 8: Rounding errors in molar mass calculations.** Use precise atomic masses: H = 1, C = 12, N = 14, O = 16, S = 32, Ca = 40, Cl = 35.5. Small rounding can shift your answer away from the correct option (e.g., 74 g/mol vs. 73 g/mol for Ca(OH)₂).

**Pre-Exam Preparation (2–3 weeks before):** 1. **Sort by difficulty.** Identify which topics you find hardest: Laws of Chemical Combination, mole concept, or formula writing. Allocate extra revision time to these. 2. **Build a quick-reference card.** Write down atomic masses (H, C, N, O, S, Ca, Mg, Cl), Avogadro's number (6.022 × 10²³), and molar volume (22.4 L/mol at STP). Glance at it daily until memorized. 3. **Time yourself on easy MCQs first.** Aim for 1–1.5 minutes per question on easy level. Slow timing indicates weak fundamentals. **During the Exam (20–25 minutes for 10 MCQs in Chapter 3):** 1. **Skim all 10 questions first (1–2 min).** Identify which are pure conceptual (Laws, definitions) vs. numerical (molar mass, moles). Tackle conceptual ones first—they're faster. 2. **Skip tough questions initially.** If a Q takes >2 min, move on. Come back when you've secured easy marks. 3. **Use elimination aggressively.** In assertion-reason MCQs, eliminate 'both false' if even one statement is true. Eliminate A/R if the reason is irrelevant to the assertion. 4. **Double-check numerical answers.** Recalculate once using a different method (e.g., if you used mass ÷ molar mass = moles, verify via molecules = moles × Avogadro's). 5. **Leave ~2 minutes for a final scan.** Ensure you haven't misread any subscript or decimal point. **Common Time Wasters to Avoid:** - Overthinking easy conceptual Qs (e.g., 'which is a diatomic molecule?')—trust your knowledge. - Re-reading the same Q thrice. Read once, decide, move on. - Attempting complex multi-step numericals if you can't set up the equation within 1 min. - Second-guessing correct answers due to panic. **Sample Timeline for 10 MCQs (20 min):** - Q1–Q4 (easy, pure concept): 4 min - Q5–Q7 (easy-medium, simple calculation): 5 min - Q8–Q10 (medium, assertion-reason or 2-step math): 7 min - Review & final check: 4 min **Adaptive Strategy:** If Chapter 3 is only worth 5 questions in your exam, prioritize Laws of Chemical Combination, molar mass, and mole concept—these appear most frequently. Start a 3-day free trial at cbsetutor.ai to access personalized Chapter 3 quizzes with instant feedback and adaptive difficulty scaling.

**After completing these 30 MCQs, verify your mastery:** - [ ] I can write the chemical formula of any compound given valencies of elements. - [ ] I can calculate molecular mass given atomic masses (and I avoid confusion with mass number). - [ ] I can convert between moles, grams, and number of molecules/atoms without hesitation. - [ ] I understand the Law of Conservation of Mass, Law of Definite Proportions, and Law of Multiple Proportions with real examples. - [ ] I can solve a 2-step molar problem (e.g., 5 g of a compound → moles → number of molecules) in under 2 minutes. - [ ] I never confuse Avogadro's number (6.022 × 10²³) with molar volume (22.4 L/mol) or molar mass (g/mol). **If you scored:** - **<70% (Easy):** Review NCERT Section 3.1–3.3 (Atoms & Molecules definitions). Drill formula-writing exercises daily. - **70–85% (Medium):** Your fundamentals are solid. Focus on numerical accuracy: redo Q8–Q10, 14–15, 20 type problems until you spot calculation errors instantly. - **>85% (Hard):** Ready for Board exam! Now attempt past-year question papers (2023–2024) and full-length mock tests. **Next Topics in Class 9 Chemistry:** - Chapter 4: Structure of the Atom (electron configuration, atomic number, mass number) - Chapter 5: The Fundamental Unit of Life (if Biology is your stream) These chapters build directly on Chapter 3 concepts, so ensure no gaps. Regular revision of this MCQ set weekly will cement your learning for long-term retention and Board success.