Chapter 7 — Atoms and Molecules: Multiple Choice Questions
Instructions: Each question carries 1 mark. Choose the best answer.
1. One mole of any substance contains ______ particles.
Answer
(a) 6.022 × 10²⁶ (b) 6.022 × 10²³ (c) 6.022 × 10²⁰ (d) 6.022 × 10¹⁶
✅ Answer: (b) 6.022 × 10²³
ExplanationOne mole of any substance (atoms, molecules, ions) contains 6.022 × 10²³ particles. This is called Avogadro's number (Nₐ). Named after Italian scientist Amedeo Avogadro.
2. The molar mass of water (H₂O) is ______.
Answer
(a) 10 g/mol (b) 16 g/mol (c) 18 g/mol (d) 20 g/mol
✅ Answer: (c) 18 g/mol
ExplanationMolar mass of H₂O = 2(1) + 16 = 2 + 16 = 18 g/mol (Atomic mass: H = 1, O = 16)
3. The number of moles in 44 g of CO₂ is ______.
Answer
(a) 0.5 mol (b) 1 mol (c) 2 mol (d) 44 mol
✅ Answer: (b) 1 mol
ExplanationMolar mass of CO₂ = 12 + 2(16) = 12 + 32 = 44 g/mol Number of moles = mass/molar mass = 44/44 = 1 mol
4. The formula for calculating the number of moles is ______.
Answer
(a) n = M × m (b) n = m / M (c) n = M / m (d) n = m + M
✅ Answer: (b) n = m / M
ExplanationNumber of moles (n) = mass of sample (m) / molar mass (M)
- n in mol
- m in grams
- M in g/mol
5. Which of the following represents the empirical formula of glucose (C₆H₁₂O₆)?
Answer
(a) C₆H₁₂O₆ (b) CH₂O (c) C₂H₄O₂ (d) C₃H₆O₃
✅ Answer: (b) CH₂O
ExplanationGlucose molecular formula: C₆H₁₂O₆ → ratio C:H:O = 6:12:6 = 1:2:1 Simplest ratio → Empirical formula = CH₂O (n = 6, so molecular formula = 6 × CH₂O = C₆H₁₂O₆)
6. The atomic mass of carbon is 12 u. The molar mass of carbon is ______.
Answer
(a) 6 g/mol (b) 12 g/mol (c) 24 g/mol (d) 1.2 g/mol
✅ Answer: (b) 12 g/mol
ExplanationThe numerical value of atomic mass (in u) equals the molar mass (in g/mol). Carbon atomic mass = 12 u → Molar mass = 12 g/mol. So 12 g of carbon contains 6.022 × 10²³ atoms.
7. One mole of oxygen gas (O₂) at STP occupies a volume of ______.
Answer
(a) 11.2 L (b) 22.4 L (c) 32 L (d) 44.8 L
✅ Answer: (b) 22.4 L
ExplanationAt STP (Standard Temperature and Pressure: 0°C, 1 atm), one mole of any gas occupies 22.4 litres. This is called the molar volume of a gas at STP. Valid for all ideal gases regardless of their nature.
8. The number of molecules in 2 moles of H₂ is ______.
Answer
(a) 6.022 × 10²³ (b) 12.044 × 10²³ (c) 3.011 × 10²³ (d) 18.066 × 10²³
✅ Answer: (b) 12.044 × 10²³
ExplanationNumber of molecules = n × Nₐ = 2 × 6.022 × 10²³ = 12.044 × 10²³ (= 1.2044 × 10²⁴)
9. The percentage of oxygen in water (H₂O) is ______.
Answer
(a) 11.1% (b) 44.4% (c) 88.9% (d) 55.5%
✅ Answer: (c) 88.9%
ExplanationMolar mass of H₂O = 18 g/mol; Mass of O = 16 g/mol % of O = (mass of O / molar mass) × 100 = (16/18) × 100 = 88.9% % of H = (2/18) × 100 = 11.1%
10. The law "In a chemical compound, elements are always combined in a fixed ratio by mass" is ______.
Answer
(a) Law of conservation of mass (b) Law of multiple proportions (c) Law of definite proportions (d) Avogadro's law
✅ Answer: (c) Law of definite proportions
ExplanationLaw of Definite Proportions (Proust's Law): A chemical compound always contains the same elements in the same proportion by mass, regardless of its source or method of preparation. Example: Water (H₂O) always has H:O = 1:8 by mass.
Quick Revision — Key Concepts
| Concept | Formula / Value |
|---|
| Avogadro's number | Nₐ = 6.022 × 10²³ mol⁻¹ |
| Number of moles | n = m/M |
| Number of particles | N = n × Nₐ |
| Molar volume at STP | 22.4 L/mol |
| Molar mass of H₂O | 18 g/mol |
| Molar mass of CO₂ | 44 g/mol |
| Molar mass of NaCl | 58.5 g/mol |
| Percentage composition | (mass of element / molar mass) × 100 |
Common MCQ Traps
- Molar mass ≠ atomic mass in g: Molar mass = atomic mass in g/mol (same number, different unit)
- Empirical vs molecular formula: Empirical is simplest ratio; molecular is actual
- 22.4 L at STP only for gases — not applicable to solids or liquids
- Avogadro's number for molecules: H₂O has 18 g/mol → 1 mole = 6.022 × 10²³ molecules of H₂O
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