Short answerEasy2 marks
The mass of 11.2 litres of a certain gas at s.t.p. is 24 g. Find the gram molecular mass of the gas.
Topic: Mole concept, gas volumes and stoichiometry
Previous-year question practice database
Answer
Exam answer
Given: 11.2 litres at STP has mass 24 g.
Moles = volume at STP ÷ 22.4 = 11.2 ÷ 22.4 = 0.5 mol
Gram molecular mass = mass ÷ moles = 24 ÷ 0.5 = 48 g mol⁻¹
Gram molecular mass = 48 g mol⁻¹
Explanation
11.2 litres at STP is exactly half of 22.4, so it is half a mole — recognise that and the arithmetic is instant. Half a mole weighs 24 g, so a whole mole weighs 48 g.
Dividing by 0.5 is the same as multiplying by 2, and this is where the mistake happens: students multiply by 0.5 instead and get 12. If half a mole weighs 24 g, a full mole must weigh MORE, not less.
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