JEE Main 2023 · 12 April · morning · Single correct · +4 / −1

A metal chloride contains 55.0 % of chlorine by weight . 100 mL vapours of the metal chloride at STP weigh 0.57 g. The molecular formula of the metal chloride is
(Given: Atomic mass of chlorine is 35.5 u)

  1. (a) MCl₂
  2. (b) MCl₄
  3. (c) MCl₃
  4. (d) MCl
Answer and solution

Answer (a) MCl₂

Solution

Chemistry ma’am · 3 steps

Get the molar mass from the vapour data first, then ask how many chlorines fit inside it.

  1. Molar mass. At STP one mole of vapour fills 22400 mL. If 100 mL weighs 0.57 g, a full mole weighs 224 times as much.

    M = 0.57 × 22400/100 = 127.68 g mol⁻¹

  2. Chlorine per mole. Chlorine is 55.0% of this mass, so one mole contains 0.55 × 127.68 = 70.22 g of chlorine. That is 70.22/35.5 = 1.98 ≈ 2 chlorine atoms.
  3. So. Each molecule carries two chlorine atoms, so the molecular formula is MCl₂, option (a).

The idea to keep. The percentage alone gives only a ratio. Pair it with the molar mass from vapour density or gas volume, and (mass % × molar mass) ÷ atomic mass counts the atoms per molecule.

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.