JEE Main 2022 · 28 June · evening · Single correct · +4 / −1

Compound A contains 8.7% Hydrogen, 74% Carbon and 17.3% Nitrogen. The molecular formula of the compound is,
Given : Atomic masses of C, H and N are 12, 1 and 14 amu respectively.
The molar mass of the compound A is 162 g mol⁻¹.

  1. (a) C₄H₆N₂
  2. (b) C₂H₃N
  3. (c) C₅H₇N
  4. (d) C₁₀H₁₄N₂
Answer and solution

Answer (d) C₁₀H₁₄N₂

Solution

Chemistry ma’am · 3 steps

Turn percentages into moles, find the simplest ratio, then scale it up to reach 162.

  1. Take 100 g. In 100 g of A, each percentage becomes a mass in grams. Divide each mass by its atomic mass to get moles, then divide every mole value by the smallest one.
    ElementMass in 100 gMolesDivide by 1.236
    C74 g74/12 = 6.1674.99 ≈ 5
    H8.7 g8.7/1 = 8.77.04 ≈ 7
    N17.3 g17.3/14 = 1.2361
  2. Empirical formula. The simplest whole-number ratio is C : H : N = 5 : 7 : 1, so the empirical formula is C₅H₇N. Its mass is 5 × 12 + 7 × 1 + 14 = 81 g mol⁻¹.

    n = (molar mass)/(empirical formula mass) = 162/81 = 2

  3. So. Multiply every subscript by 2: the molecular formula is C₁₀H₁₄N₂, option (d).

The idea to keep. Percentages only give you the empirical formula. Always finish by dividing the molar mass by the empirical formula mass and multiplying through.

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 2022.