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MCQ practice · SSC Chemistry Board questions · Chapter 6: Concept of Mole and Chemical Counting

Dinajpur Board 2022

After the reaction, which reactant is left over, and how much?

Think it through first, then pick an option.

  1. a 2.143 gm H₂
  2. b 0.857 gm H₂
  3. c 4 gm Cl₂
  4. d 14 gm Cl₂
Answer and explanation

Correct answer: b 0.857 gm H₂

N₂ = 10/28 = 0.357 mole (limiting) needs 3 × 0.357 = 1.071 mole H₂ = 2.143 g; 3 − 2.143 = 0.857 g H₂ is left.

Why the other options are wrong

  • a 2.143 gm H₂: 2.143 g is the H₂ that reacts; the leftover is 3 − 2.143 = 0.857 g.
  • c 4 gm Cl₂: There is no Cl₂ in this reaction; H₂ is left over, 0.857 g.
  • d 14 gm Cl₂: There is no Cl₂ in the reaction, and N₂, being limiting, is used up; 0.857 g of H₂ is left.