NumericalModerate2 marks
1(e)(i)
Calculate:
The number of moles in 12g of oxygen gas. [O = 16]
Topic: Mole concept, gas volumes and stoichiometry
Previous-year question practice database
Answer
Exam answer
Given: mass of oxygen gas = 12 g. [O = 16]
Molar mass of O₂ = 2 × 16 = 32 g mol⁻¹
Moles = mass ÷ molar mass = 12 ÷ 32 = 0.375 mol
Number of moles of oxygen gas = 0.375 mol
Explanation
The question says oxygen GAS, so the particle is O₂ and the molar mass is 32. Dividing by 16 gives 0.75 mol — exactly double, and the single most common mistake in this chapter.
Read the wording each time. "Oxygen gas", "a molecule of oxygen" and "12 g of O₂" all mean 32. "Gram atoms of oxygen" or "oxygen atoms" would mean 16.
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