JEE Main 2023 · 31 January · evening · Single correct · +4 / −1

When a hydrocarbon A undergoes complete combustion it requires 11 equivalents of oxygen and produces 4 equivalents of water. What is the molecular formula of A ?

  1. (a) C₉ H₈
  2. (b) C₅ H₈
  3. (c) C₁₁ H₄
  4. (d) C₁₁ H₈
Answer and solution

Answer (a) C₉ H₈

Solution

Chemistry ma’am · 4 steps

Water fixes the hydrogens first. Whatever oxygen is left over must be burning carbon.

  1. General equation. Write the hydrocarbon as CₓH_y. Each C becomes one CO₂ and needs one O₂. Every 2 H make one H₂O and need half an O₂.

    CₓH_y + (x + y/4)O₂ → x CO₂ + y/2 H₂O

  2. Hydrogen from water. 4 moles of water means y/2 = 4, so y = 8. These 8 H atoms use 8/4 = 2 moles of O₂.
  3. Carbon from oxygen. The total oxygen is x + y/4 = 11, so x + 2 = 11 and x = 9.
  4. So. The hydrocarbon is C₉H₈, option (a). Check: C₉H₈ + 11O₂ → 9CO₂ + 4H₂O balances.

The idea to keep. For CₓH_y, the O₂ needed is x + y/4. Get y from the water first, then the leftover oxygen tells you the carbons.

Answer from the NTA final answer key. Solution by PrepWiser.

Study Mole Concept & Stoichiometry

Asked about 210 times in JEE Main since 2020. This question is from 2023.