2023 O Level Chem P1 Q15 — Empirical Formula Solution

Answer: C. Divide each percentage by the relevant relative atomic mass to get mole ratios:

Co: \(48.2 / 59 = 0.817\) mol
P: \(16.9 / 31 = 0.545\) mol
O: \(34.9 / 16 = 2.181\) mol

Divide all by the smallest (0.545):

Co: \(0.817 / 0.545 = 1.500\)
P: \(0.545 / 0.545 = 1.000\)
O: \(2.181 / 0.545 = 4.000\)

The ratio Co : P : O = 1.5 : 1 : 4. Multiply through by 2 to clear the half: 3 : 2 : 8. The empirical formula is \(\text{Co}_3\text{P}_2\text{O}_8\) — i.e. \(\text{Co}_3(\text{PO}_4)_2\), cobalt(II) phosphate — which is option C.

Option A (\(\text{Co}_2\text{P}_4\text{O}\)) reverses the Co:P ratio and gives the wrong oxygen count. Option B (\(\text{Co}_3\text{PO}_2\)) has too few O and one too few P. Option D (\(\text{Co}_5\text{P}_7\text{O}_2\)) doesn't match any simple multiple of the calculated 3:2:8 ratio.