Atoms and Molecules – MCQs (Practice Set)

Q1. Which law states that "mass can neither be created nor destroyed in a chemical reaction"?
(a) Law of constant proportion
(b) Law of conservation of mass
(c) Dalton’s atomic theory
(d) Law of multiple proportion

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Q2. Who gave the law of constant proportions?
(a) Dalton
(b) Lavoisier
(c) Proust
(d) Rutherford

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Q3. The relative atomic mass of an element is defined as:
(a) Mass of one atom compared with 1/16th of O atom
(b) Mass of one atom compared with 1/12th of C-12 atom
(c) Actual mass of an atom
(d) Mass number of atom

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Q4. The symbol "u" in atomic mass stands for:
(a) Unit
(b) Unified mass
(c) Uranium
(d) Universal constant

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Q5. Which of the following is the correct chemical formula of magnesium chloride?
(a) MgCl
(b) Mg₂Cl
(c) MgCl₂
(d) Mg₂Cl₂

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Q6. Which of the following is not a molecule of an element?
(a) H₂
(b) O₂
(c) H₂O
(d) N₂

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Q7. The formula of sodium carbonate is:
(a) Na₂CO₃
(b) NaCO₃
(c) Na₃CO₂
(d) NaC₂O₃

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Q8. Which one is a polyatomic ion?
(a) Na⁺
(b) SO₄²⁻
(c) Cl⁻
(d) Mg²⁺

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Q9. The mass of one mole of oxygen molecules (O₂) is:
(a) 16 g
(b) 18 g
(c) 32 g
(d) 8 g

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Q10. Which of the following has both ionic and covalent bonds?
(a) HCl
(b) NaOH
(c) H₂O
(d) Cl₂

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Q11. One mole of any substance contains:
(a) 6.022 × 10²³ atoms only
(b) 6.022 × 10²³ molecules only
(c) 6.022 × 10²³ particles
(d) Mass number of the element

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Q12. Which of the following represents a diatomic molecule?
(a) O₃
(b) H₂O
(c) N₂
(d) CO₂

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Q13. Which law is illustrated by the reaction:
2H₂ + O₂ → 2H₂O
(a) Law of constant proportion
(b) Law of conservation of mass
(c) Law of multiple proportion
(d) Both (a) and (b)

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Q14. The chemical formula of aluminium oxide is:
(a) Al₂O₃
(b) AlO
(c) AlO₂
(d) Al₂O

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Q15. Which of the following is triatomic?
(a) H₂
(b) H₂O
(c) O₃
(d) Both (b) and (c)

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Q16. Which of the following is not correctly matched?
(a) Ammonium ion – NH₄⁺
(b) Nitrate ion – NO₃⁻
(c) Carbonate ion – CO₄²⁻
(d) Sulphate ion – SO₄²⁻

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Q17. The number of atoms in one molecule of Ca(OH)₂ is:
(a) 3
(b) 5
(c) 4
(d) 2

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Q18. Which of the following gases confirms the law of definite proportion when reacted with hydrogen to form water?
(a) Oxygen
(b) Nitrogen
(c) Carbon dioxide
(d) Chlorine

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Q19. Which is the smallest particle that takes part in a chemical reaction?
(a) Atom
(b) Molecule
(c) Electron
(d) Proton

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Q20. The chemical formula of baking soda is:
(a) Na₂CO₃
(b) NaHCO₃
(c) Na₂SO₄
(d) NaOH