Compounds of the alkali metals
2 marks · EasyA student heats a piece of potassium weighing 3.9 g in air. Potassium reacts fully to form potassium oxide (K$_2$O) according to the equation:
Calculate the maximum mass of potassium oxide formed from 3.9 g of potassium. (Relative atomic masses: K = 39, O = 16)
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4.7 g (±0.1)
Also accept: 4.6875 g
Common mistakes: Forgetting to double the oxygen in molar mass of K$_2$O
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Marking points
M1 · 1 mark · Converts mass of potassium to moles and uses stoichiometry to find moles of K$_2$O.
A1 · 1 mark · Correctly calculates the mass of potassium oxide (allow error carried through from moles). (follow-through)
Worked solution
Moles of potassium = $\frac{3.9}{39} = 0.1$ mol.
From the equation: 4K makes 2K$_2$O, so 4 mol K makes 2 mol K$_2$O. So 0.1 mol K makes 0.05 mol K$_2$O.
Mr of K$_2$O = $2 \times 39 + 16 = 94$.
Mass of K$_2$O formed = $0.05 \times 94 = 4.7$ g.