Equal volumes of ammonia gas and chlorine gas are kept in two different containers under the same conditions of temperature and pressure. Find the number of molecules contained in chlorine gas when the mass of ammonia is 34 g. [Atomic weight: Cl=35.5, H=1, N=14]
Topic: Mole concept, gas volumes and stoichiometry
Answer
Given: equal volumes of NH₃ and Cl₂ at the same temperature and pressure; mass of NH₃ = 34 g. [N = 14, H = 1]
Molar mass of NH₃ = 14 + (3 × 1) = 17 g mol⁻¹
Moles of NH₃ = 34 ÷ 17 = 2 mol
Number of NH₃ molecules = 2 × 6.022 × 10²³ = 1.2044 × 10²⁴
By Avogadro's law, equal volumes of gases at the same temperature and pressure contain equal numbers of molecules, so the chlorine contains the same number.
Number of Cl₂ molecules = 1.2044 × 10²⁴
The whole question turns on Avogadro's law. Find the number of ammonia molecules, and the chlorine — same volume, same conditions — has exactly the same number.
This is why the atomic mass of chlorine is given but never used. It is a distractor. Students who use it, converting 34 g into moles of Cl₂ by dividing by 71, get 0.48 mol and the wrong answer. The chlorine in that container weighs far more than 34 g — but it is the same count of molecules.