Chemical Laws and Mole Concept – NDA Chemistry PYQs

Q1. How many gram atoms of hydrogen are present in 0.04 mole of CuSO₄·5H₂O?
(a) 0.04 (b) 0.2 (c) 0.4 (d) 4

Answer: (c) 0.4
CuSO₄·5H₂O has 5 water molecules. Each water has 2 hydrogen atoms. So each formula unit has 10 hydrogen atoms. 0.04 mole × 10 = 0.4 gram atoms of hydrogen.
[NDA 2006-I]

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Q2. One litre of an unknown gas weighs 1.25 g at STP. Which one of the following can be the formula for the gas?
(a) O₂ (b) CO (c) CO₂ (d) SO₂

Answer: (b) CO
At STP, 1 mole of gas = 22.4 litres. Mass of 22.4 litres = 22.4 × 1.25 = 28 g/mol. Molecular mass = 28. CO = 12+16 = 28. That matches. O₂ = 32, CO₂ = 44, SO₂ = 64 — none match.
[NDA 2006-I]

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Q3. Which one of the following elements shows variable equivalent mass?
(a) Zinc (b) Silver (c) Calcium (d) Iron

Answer: (d) Iron
Iron can be Fe²⁺ (valency 2) or Fe³⁺ (valency 3). Because valency changes, equivalent mass changes too. Zinc, silver, and calcium each have only one valency — so their equivalent mass is fixed.
[NDA 2007-I]

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Q4. The atomic weights are expressed in atomic mass units. Which one of the following is used as a standard?
(a) ¹H₁ (b) ¹²C₆ (c) ¹⁶O₈ (d) ³⁵Cl₁₇

Answer: (b) ¹²C₆
Carbon-12 is the internationally accepted standard for atomic mass. One atomic mass unit (u) is defined as exactly 1/12th the mass of one carbon-12 atom.
[NDA 2007-I]

Q5. The molecular weight and equivalent weight of which one of the following is the same?
(a) H₂SO₄ (b) KMnO₄ (c) H₂C₂O₄ (d) NaOH

Answer: (d) NaOH
NaOH has molecular weight 40 and acidity 1 (one replaceable OH⁻). Equivalent weight = 40/1 = 40 = molecular weight. H₂SO₄ has basicity 2, so equivalent weight = 49. KMnO₄ and H₂C₂O₄ also have equivalent weights different from molecular weight.
[NDA 2008-I]

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Q6. What is the number of water molecules present in a tiny drop of water (volume 0.0018 mL) at room temperature?
(a) 4.84 × 10¹⁷ (b) 4.184 × 10¹⁸ (c) 6.023 × 10¹⁹ (d) 6.023 × 10²³

Answer: (c) 6.023 × 10¹⁹
0.0018 mL of water = 0.0018 g. Moles = 0.0018/18 = 10⁻⁴ mol. Molecules = 10⁻⁴ × 6.023 × 10²³ = 6.023 × 10¹⁹.
[NDA 2008-II]

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Q7. How many hydrogen molecules are required to react with 100 molecules of nitrogen?
N₂ + 3H₂ → 2NH₃
(a) 100 (b) 200 (c) 300 (d) 400

Answer: (c) 300
From the balanced equation, 1 molecule of N₂ requires 3 molecules of H₂. So 100 molecules of N₂ require 100 × 3 = 300 molecules of H₂.
[NDA 2009-I]

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Q8. Which of the following is the best example of the law of conservation of mass?
(a) When 12 g of carbon is heated in vacuum, there is no change in mass
(b) Weight of platinum wire is the same before and after heating
(c) A sample of air increases in volume when heated at constant pressure but mass remains unchanged
(d) 12 g of carbon combines with 32 g of oxygen to give 44 g of carbon dioxide

Answer: (d) 12 g of carbon combines with 32 g of oxygen to give 44 g of carbon dioxide
This is the only option where a chemical reaction takes place and the total mass is conserved. 12 + 32 = 44. The other options involve physical changes — not chemical reactions.
[NDA 2009-II]

Q9. Which law is confirmed by the fact that limestone from any part of the world always has the same composition?
(a) Law of conservation of mass
(b) Law of definite proportion
(c) Law of multiple proportions
(d) Avogadro’s law

Answer: (b) Law of definite proportion
The law of definite proportions says a pure compound always has the same composition by mass — regardless of its source. Limestone (CaCO₃) from India, France, or Australia always has the same ratio of calcium, carbon, and oxygen.
[NDA 2010-II]

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Q10. The proposition “equal volumes of different gases contain equal numbers of molecules at the same temperature and pressure” is known as?
(a) Avogadro’s hypothesis (b) Gay-Lussac’s hypothesis (c) Planck’s hypothesis (d) Kirchhoff’s theory

Answer: (a) Avogadro’s hypothesis
This is the exact statement of Avogadro’s law — equal volumes of gases at the same temperature and pressure contain equal numbers of molecules. It was proposed by Amedeo Avogadro in 1811.
[NDA 2017-II]

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Q11. Equal volume of all gases, when measured at the same temperature and pressure, contain an equal number of particles. Who proposed the above law?
(a) Charles (b) Boyle (c) Avogadro (d) Lussac

Answer: (c) Avogadro
This is Avogadro’s law. Charles proposed the law relating volume and temperature. Boyle proposed the law relating pressure and volume. Gay-Lussac proposed the law of combining volumes.
[NDA 2023-I]

Q12. Which one of the following statements about the law of conservation of mass is correct?
(a) A given compound always contains exactly the same proportion of elements
(b) When gases combine in a reaction, they do so in a simple ratio by volume, provided all gases are at room temperature
(c) Matter can neither be created nor destroyed
(d) Equal volumes of all gases at same temperature and pressure contain equal number of molecules

Answer: (c) Matter can neither be created nor destroyed
Option (a) is the law of definite proportions (Proust). Option (b) is the law of combining volumes (Gay-Lussac). Option (d) is Avogadro’s law. Only option (c) correctly states the law of conservation of mass.
[NDA 2018-II]

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Q13. How many moles of hydrogen atoms are present in one mole of Aluminium hydroxide?
(a) One mole (b) Two moles (c) Three moles (d) Four moles

Answer: (c) Three moles
Al(OH)₃ has 3 hydroxide groups. Each has 1 hydrogen atom. So one formula unit has 3 hydrogen atoms. One mole of Al(OH)₃ contains 3 moles of hydrogen atoms.
[NDA 2016-II]

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Q14. The mass of 0.5 mole of N₂ gas is:
(a) 7 g (b) 14 g (c) 21 g (d) 28 g

Answer: (b) 14 g
Molar mass of N₂ = 14 + 14 = 28 g/mol. Mass of 0.5 mole = 0.5 × 28 = 14 g.
[NDA 2024-II]

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Q15. Which one of the following is the correct molecular formula of ammonium carbonate if the valency of ammonium ion is +1 and carbonate anion is −2?
(a) (NH₄)₂CO₃ (b) NH₄(CO₃)₂ (c) (NH₃)₂CO₃ (d) NH₄CO₃

Answer: (a) (NH₄)₂CO₃
Cross multiply the valencies. NH₄⁺ has valency 1 → subscript of CO₃ becomes 1. CO₃²⁻ has valency 2 → subscript of NH₄ becomes 2. Formula = (NH₄)₂CO₃.
[NDA 2022-I]

Q16. Which one among the following is the equivalent weight of sulphuric acid?
(Atomic weight: H=1, S=32, O=16)
(a) 98 (b) 60 (c) 49 (d) 100

Answer: (c) 49
Molecular weight of H₂SO₄ = 2(1) + 32 + 4(16) = 98 g/mol. H₂SO₄ has basicity 2 (two replaceable H⁺ ions). Equivalent weight = 98/2 = 49 g/mol.
[NDA 2012-II]

Q18. Avogadro’s law states that equal volumes of all gases at the same temperature and pressure contain equal numbers of molecules. This law applies to:
(a) Gases only (b) Liquids only (c) Solids only (d) All states of matter

Answer: (a) Gases only
Avogadro’s law is specifically for gases — it describes the behaviour of gas molecules. It does NOT apply to solids or liquids. Solid carbon and gaseous oxygen at the same volume and conditions do NOT contain equal numbers of particles.
[NDA 2010-II]