Short answerEasy1 mark
5(b)
A gas cylinder can hold 1 kg of hydrogen at room temperature and pressure:
Find the number of moles of hydrogen present.
Topic: Mole concept, gas volumes and stoichiometry
Previous-year question practice database
Answer
Exam answer
Given: mass of hydrogen = 1 kg = 1000 g.
Molar mass of H₂ = 2 × 1 = 2 g mol⁻¹
Moles = mass ÷ molar mass = 1000 ÷ 2 = 500 mol
Number of moles of hydrogen = 500 mol
Explanation
Convert kilograms to grams first — molar mass is in grams per mole, so the mass must be in grams. Leaving it as 1 kg and dividing by 2 gives 0.5, out by a factor of a thousand.
Use 2, not 1. Hydrogen gas is H₂, a diatomic molecule. Using the atomic mass 1 doubles the answer to 1000 mol.
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