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Laws of chemical combination; atomic and molecular masses: NEET MCQs with answers

89 questions on this topic of Some Basic Concepts in Chemistry (NEET 2027 syllabus). Here are 12; tap one to see its answer and explanation.

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easy

1. Given below are two statements: Statement I: Equal volumes of all gases under identical conditions of temperature and pressure contain an equal number of molecules. Statement II: Dalton's atomic theory was successful in explaining Gay-Lussac's Law of Gaseous Volumes without modification. In the light of the above statements, choose the most appropriate answer from the options given below:

  1. Both Statement I and Statement II are correct
  2. Statement I is correct but Statement II is incorrect
  3. Statement I is incorrect but Statement II is correct
  4. Both Statement I and Statement II are incorrect
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Answer: (B) Statement I is correct but Statement II is incorrect

Statement I is Avogadro's Law, which is correct. Statement II is incorrect because Dalton's atomic theory could not explain the law of combining volumes; it was Avogadro who modified the concept by distinguishing between atoms and molecules.

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2. Naturally occurring chlorine consists of two isotopes, ³⁵Cl (atomic mass = 34.97 u) and ³⁷Cl (atomic mass = 36.97 u). If the average atomic mass of chlorine is 35.45 u, what is the approximate relative abundance of the ³⁷Cl isotope?

  1. 76%
  2. 35%
  3. 24%
  4. 12%
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Answer: (C) 24%

Let the fractional abundance of ³⁷Cl be x. Then 34.97(1 - x) + 36.97x = 35.45. Simplifying gives 34.97 + 2.00x = 35.45, so 2.00x = 0.48, which yields x = 0.24 or 24%.

hard

3. Three elements P, Q, and R combine to form compounds PQ, PR, and QR. In PQ, the mass ratio of P to Q is 1 : 2. In PR, the mass ratio of P to R is 1 : 4. If Q and R combine to form QR, what simple integral ratio of their combining masses (Q : R) directly illustrates the Law of Reciprocal Proportions?

  1. 4 : 1
  2. 2 : 1
  3. 1 : 4
  4. 1 : 2
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Answer: (D) 1 : 2

In PQ, 1 part by mass of P combines with 2 parts by mass of Q. In PR, 1 part by mass of P combines with 4 parts by mass of R. The ratio of masses of Q and R combining with a fixed mass (1 part) of P is 2 : 4 = 1 : 2. According to the Law of Reciprocal Proportions, when Q and R combine directly, their combining masses will be in the ratio 1 : 2 (or a simple multiple thereof).

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4. Given below are two statements: one is labelled as Assertion (A) and the other is labelled as Reason (R). Assertion (A): Berzelius proposed that equal volumes of all gases under identical conditions of temperature and pressure contain an equal number of atoms. Reason (R): Berzelius's hypothesis was modified by Avogadro by drawing a clear distinction between atoms and molecules. In light of the above statements, choose the correct answer from the options given below:

  1. Both (A) and (R) are true and (R) is the correct explanation of (A)
  2. Both (A) and (R) are true but (R) is not the correct explanation of (A)
  3. (A) is true but (R) is false
  4. (A) is false but (R) is true
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Answer: (B) Both (A) and (R) are true but (R) is not the correct explanation of (A)

J.J. Berzelius suggested that equal volumes of all gases under the same conditions contain equal numbers of atoms, which led to fractional atoms in reactions. Avogadro resolved this issue by differentiating between atoms and molecules, proposing that equal volumes contain equal numbers of molecules. Both statements are factually correct, but (R) is the historical resolution/modification of the hypothesis rather than the explanation of why Berzelius originally made his proposition.

easy

5. Which of the following historical statements regarding Dalton's atomic theory correctly represents one of its original postulates?

  1. Atoms of different elements always combine in fractional ratios to form compounds.
  2. Atoms of the same element can have different masses and chemical properties.
  3. Chemical reactions involve reorganization of atoms; these are neither created nor destroyed in a chemical reaction.
  4. Matter consists of divisible particles called atoms.
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Answer: (C) Chemical reactions involve reorganization of atoms; these are neither created nor destroyed in a chemical reaction.

According to Dalton's atomic theory, chemical reactions involve reorganization of atoms, and atoms are neither created nor destroyed during a chemical reaction. Matter consists of indivisible atoms, and atoms of a given element have identical mass and properties.

hard

6. Given below are two statements: Statement I: Avogadro proposed that equal volumes of all gases at the same temperature and pressure should contain equal number of molecules. Statement II: Dalton believed that atoms of the same element could not combine with each other to form diatomic molecules. In the light of the above statements, choose the correct answer:

  1. Both Statement I and Statement II are correct
  2. Statement I is correct but Statement II is incorrect
  3. Statement I is incorrect but Statement II is correct
  4. Both Statement I and Statement II are incorrect
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Answer: (A) Both Statement I and Statement II are correct

Both statements are correct. Avogadro's hypothesis (1811) proposed equal volumes containing equal numbers of molecules. Dalton rejected the idea of diatomic elemental molecules (like O₂ or H₂) because he assumed similar atoms would repel each other, which prevented him from arriving at Avogadro's hypothesis.

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7. What total volume of gaseous products is obtained at the same temperature and pressure when 24 mL of methane (CH₄) gas undergoes complete combustion with excess oxygen gas according to the equation: CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(g)?

  1. 24 mL
  2. 48 mL
  3. 72 mL
  4. 96 mL
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Answer: (C) 72 mL

According to Gay-Lussac's Law of Combining Volumes, gaseous reactants and products react in simple whole-number ratios of their volumes. 1 volume of CH₄ yields 1 volume of CO₂(g) and 2 volumes of H₂O(g), totaling 3 volumes of gaseous products. Therefore, 24 mL of CH₄ produces 24 × 3 = 72 mL of gaseous products.

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8. Which of the following sets of compounds correctly demonstrates the Law of Multiple Proportions?

  1. NaCl and NaBr
  2. SnCl₂ and SnCl₄
  3. H₂S and SO₂
  4. CO₂ and CS₂
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Answer: (B) SnCl₂ and SnCl₄

The Law of Multiple Proportions applies when two elements combine to form more than one compound. SnCl₂ and SnCl₄ are formed from the same two elements (tin and chlorine), and the masses of chlorine that combine with a fixed mass of tin are in the simple integer ratio 2 : 4 = 1 : 2.

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9. Phosphorus forms two fluorides: PF₃ and PF₅. In PF₃, 31.0 g of phosphorus is combined with 57.0 g of fluorine. How much fluorine will combine with 15.5 g of phosphorus in PF₅?

  1. 95.0 g
  2. 19.0 g
  3. 28.5 g
  4. 47.5 g
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Answer: (D) 47.5 g

In PF₅, 1 mole of P (31.0 g) combines with 5 moles of F (5 × 19.0 g = 95.0 g). For 15.5 g of phosphorus (which is half a mole, 31.0 / 2), the mass of fluorine required is 95.0 / 2 = 47.5 g.

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10. Carbon combines with hydrogen to form two hydrocarbons: methane (CH₄) and ethene (C₂H₄). In CH₄, 12 g of carbon is combined with 4 g of hydrogen. In C₂H₄, 12 g of carbon is combined with 2 g of hydrogen. What is the ratio of masses of hydrogen that combine with a fixed mass (12 g) of carbon in these two compounds?

  1. 1 : 2
  2. 3 : 2
  3. 1 : 4
  4. 2 : 1
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Answer: (D) 2 : 1

In CH₄, 12 g of carbon combines with 4 g of hydrogen. In C₂H₄ (24 g C : 4 g H), 12 g of carbon combines with 2 g of hydrogen. The ratio of masses of hydrogen is 4 g : 2 g = 2 : 1, which is a simple whole-number ratio.

hard

11. Boron has two stable isotopes, ¹⁰B (atomic mass = 10.01 u) and ¹¹B (atomic mass = 11.01 u). If the standard atomic mass of boron is 10.81 u, what is the fraction of ¹⁰B atoms in naturally occurring boron?

  1. 0.80
  2. 0.10
  3. 0.20
  4. 0.25
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Answer: (C) 0.20

Let 'x' be the mole fraction of ¹⁰B. Then (1 - x) is the fraction of ¹¹B. 10.01x + 11.01(1 - x) = 10.81 => 11.01 - x = 10.81 => x = 0.20.

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12. When 20 mL of a gaseous oxide of nitrogen is mixed with excess hydrogen gas and passed over a heated catalyst, it is reduced completely to form 20 mL of N₂ gas and liquid water at the same temperature and pressure. The molecular formula of the oxide is:

  1. NO
  2. N₂O
  3. NO₂
  4. N₂O₃
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Answer: (B) N₂O

According to Avogadro's law and Gay Lussac's law, volume ratio equals mole ratio under identical conditions of temperature and pressure. Here, 20 mL (1 volume) of NₓOᵧ yields 20 mL (1 volume) of N₂. By conservation of nitrogen atoms: 1 × x = 1 × 2, so x = 2. Among the options with 2 nitrogen atoms, N₂O produces N₂ and H₂O upon reduction: N₂O(g) + H₂(g) → N₂(g) + H₂O(l), where 1 volume of N₂O gives 1 volume of N₂.

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