NumericalModerate1 mark
5(a)
A gas cylinder contains 12 x 1024 molecules of oxygen gas. If Avogadro's number is 6 x 1023; Calculate:
the volume of oxygen at S.T.P. present in the cylinder. [O=16]
Topic: Mole concept, gas volumes and stoichiometry
Previous-year question practice database
Answer
Exam answer
From part (i), moles of O₂ = 20 mol.
At STP, 1 mole of any gas occupies 22.4 litres.
Volume = moles × 22.4 = 20 × 22.4 = 448 litres
Volume of oxygen at STP = 448 litres
Explanation
Notice you do not need the mass from part (i) at all. Volume at STP depends only on the number of moles, never on which gas it is — 20 moles of hydrogen would also occupy 448 litres. That is Avogadro's law.
A student who goes the long way round — 640 g, then divide by 32, then multiply by 22.4 — gets the same answer but wastes time and invites arithmetic slips. Go straight from moles to volume.
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