Zinc reacts with hydrochloric acid to give hydrogen and zinc chloride. The volume of hydrogen gas produced at STP from the reaction of 11.5 g of zinc with excess HCl is __________ L (Nearest integer)
(Given : Molar mass of Zn is 65.4 g mol⁻¹ and Molar volume of H₂ at STP=22.7 L )

Solution

Chemistry ma’am · 3 steps

One zinc gives one hydrogen molecule, so find moles of Zn and multiply by the molar volume.

1. Balanced equation. Zn + 2HCl → ZnCl₂ + H₂. Each mole of zinc gives exactly one mole of H₂. HCl is in excess, so zinc decides how much gas forms.
2. Moles of zinc. n_(Zn) = 11.5/65.4 = 0.1758 mol, so n_(H₂) = 0.1758 mol too.

   V_(H₂) = 0.1758 mol × 22.7 L mol⁻¹ = 3.99 L

3. So. About 3.99 L of hydrogen forms at STP, which rounds to 4.

The common slip: 8. Reading the 2 in front of HCl as 2 mol of H₂ per mol of Zn doubles the volume and gives 8, not 4.

The idea to keep. In stoichiometry, go grams to moles, use the mole ratio from the balanced equation, then moles to volume. Always use the molar volume the question gives you.

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