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Empirical/molecular formulae, chemical equations and stoichiometry: NEET MCQs with answers

52 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. A hydrocarbon contains 80% carbon and 20% hydrogen by mass. If its molecular mass is 30 u, what is its molecular formula? (Atomic masses: C = 12 u, H = 1 u)

  1. CH₄
  2. C₂H₄
  3. C₂H₆
  4. C₃H₈
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Answer: (C) C₂H₆

Molar ratio of C : H = (80/12) : (20/1) = 6.67 : 20 = 1 : 3. The empirical formula is CH₃, with an empirical formula mass of 12 + 3(1) = 15 u. The ratio n = Molecular mass / Empirical formula mass = 30 / 15 = 2. Therefore, the molecular formula is (CH₃)₂ = C₂H₆.

medium

2. Given the statements: Statement I: The limiting reagent in a chemical reaction is the reactant that is present in the lowest mass. Statement II: Stoichiometric calculations are always based on the amount of limiting reagent consumed. 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.
Show answer and explanation

Answer: (C) Statement I is incorrect but Statement II is correct.

Statement I is incorrect because the limiting reagent is determined by the stoichiometric mole ratios of the balanced reaction, not solely by the initial mass. Statement II is correct because the limiting reagent is completely consumed first and therefore dictates the theoretical maximum amount of products formed.

hard

3. A gaseous hydrocarbon contains 85.7% carbon by mass. When 0.1 mole of this hydrocarbon is burnt completely in excess O₂, 8.96 L of CO₂ at STP is collected. What is the molecular formula of the hydrocarbon?

  1. C₂H₄
  2. C₃H₆
  3. C₄H₈
  4. C₅H₁₀
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Answer: (C) C₄H₈

Moles of CO₂ formed = 8.96 / 22.4 = 0.4 mol. Since 0.1 mol of hydrocarbon produces 0.4 mol of CO₂, each molecule contains 0.4 / 0.1 = 4 carbon atoms. The mass percentage of C is 85.7% (which corresponds to empirical formula CH₂). With 4 carbons, the molecular formula must be C₄H₈.

medium

4. What mass of pure oxygen gas is required for the complete combustion of 5.8 g of butane (C₄H₁₀)? (Molar mass of butane = 58 g/mol)

  1. 20.8 g
  2. 16.0 g
  3. 10.4 g
  4. 32.0 g
Show answer and explanation

Answer: (A) 20.8 g

The balanced reaction is 2C₄H₁₀ + 13O₂ → 8CO₂ + 10H₂O. Moles of C₄H₁₀ = 5.8 g / 58 g/mol = 0.10 mol. According to stoichiometry, moles of O₂ required = 0.10 × (13/2) = 0.65 mol. Mass of O₂ = 0.65 mol × 32 g/mol = 20.8 g.

easy

5. The mass of CO₂ formed when 36 g of carbon burns completely in excess oxygen is:

  1. 9.82 g
  2. 132 g
  3. 96 g
  4. 36 g
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Answer: (B) 132 g

C + O₂ → CO₂: 12 g C gives 44 g CO₂, so 36 g gives 36 × 44/12 = 132 g.

hard

6. 10 mL of a gaseous hydrocarbon requires 30 mL of oxygen for complete combustion and produces 20 mL of CO₂ under identical conditions of temperature and pressure. The molecular formula of the hydrocarbon is:

  1. C₃H₆
  2. C₂H₆
  3. C₂H₄
  4. C₃H₈
Show answer and explanation

Answer: (C) C₂H₄

For combustion of C_xH_y: C_xH_y + (x + y/4)O₂ → x CO₂ + (y/2)H₂O. Using Gay-Lussac's Law: 1 volume of C_xH_y gives x volumes of CO₂. Thus, 10x = 20 ⇒ x = 2. The volume of O₂ consumed is 10(x + y/4) = 30 ⇒ 2 + y/4 = 3 ⇒ y/4 = 1 ⇒ y = 4. Hence, the formula is C₂H₄.

easy

7. A metal oxide contains 60% metal by mass. If the equivalent mass of the metal is E, what is the value of E?

  1. 40
  2. 16
  3. 24
  4. 12
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Answer: (D) 12

Equivalent mass of metal = (mass of metal / mass of oxygen) × 8. Here, mass of metal = 60 g and mass of oxygen = 100 - 60 = 40 g. Thus, E = (60 / 40) × 8 = 12.

easy

8. Consider the unbalanced chemical equation: a FeS₂ + b O₂ → c Fe₂O₃ + d SO₂. When balanced with the smallest whole-number coefficients, what is the sum of the stoichiometric coefficients (a + b + c + d)?

  1. 19
  2. 25
  3. 23
  4. 21
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Answer: (B) 25

Balancing the equation: 4 FeS₂ + 11 O₂ → 2 Fe₂O₃ + 8 SO₂. The coefficients are a = 4, b = 11, c = 2, and d = 8. Their sum is 4 + 11 + 2 + 8 = 25.

easy

9. The mass of CO₂ formed when 18 g of carbon burns completely in excess oxygen is:

  1. 18 g
  2. 48 g
  3. 66 g
  4. 4.91 g
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Answer: (C) 66 g

C + O₂ → CO₂: 12 g C gives 44 g CO₂, so 18 g gives 18 × 44/12 = 66 g.

easy

10. An organic compound contains 75% carbon and 25% hydrogen by mass. The molecular mass of the compound is found to be 16 g/mol. What is its molecular formula? (Atomic masses: C = 12 u, H = 1 u)

  1. C₂H₆
  2. C₂H₄
  3. CH₄
  4. CH₂
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Answer: (C) CH₄

Relative moles of C = 75/12 = 6.25 and H = 25/1 = 25. The simplest molar ratio C:H is 6.25/6.25 : 25/6.25 = 1 : 4. The empirical formula is CH₄ with an empirical formula mass of 16 g/mol. Since the molecular mass is 16 g/mol, the molecular formula is CH₄.

easy

11. Given the following two statements regarding chemical stoichiometry: Statement I: The limiting reactant determines the maximum theoretical yield of the products. Statement II: The reactant present in the smallest mass is always the limiting reactant. Choose the correct option:

  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
Show answer and explanation

Answer: (B) Statement I is correct but Statement II is incorrect

Statement I is correct because the reaction ceases when the limiting reactant is completely consumed, fixing the product yield. Statement II is incorrect because the limiting reactant depends on the molar ratio and stoichiometric coefficients, not solely on the initial mass.

medium

12. Consider the following statements regarding a chemical reaction: Statement I: The stoichiometric coefficients in a balanced chemical equation represent the mole ratio in which the reactants combine and products are formed. Statement II: The actual amount of product obtained in a reaction is always equal to the theoretical amount predicted by stoichiometry. 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.
Show answer and explanation

Answer: (B) Statement I is correct but Statement II is incorrect.

Statement I is correct because stoichiometric coefficients give the molar ratios (or molecular ratios) of reactants and products in a balanced equation. Statement II is incorrect because experimental yields (actual yields) are almost always less than theoretical yields due to incomplete reactions, side reactions, or mechanical losses.

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