Home › TYS Solutions › O Level Chem › 2025 › P1 Q12

2025 O Level Chem P1 Q12 — Stoichiometry / Reacting Masses Solution

Stoichiometry / Reacting Masses 2025 O Level Pure Chemistry · Paper 1 · Q12

Question

One phosphorus pentachloride molecule, PCl₅, reacts with one ethanol molecule. The products of this reaction are one chloroethane molecule, one hydrogen chloride molecule and one molecule of product X. What is the percentage mass of chlorine in product X?

A63%
B69%
C77%
D90%

Correct answer: B

Video Solution

Explanation

Answer: B. Use conservation of atoms to identify product X.

Reactants: \(\text{PCl}_5\) and \(\text{C}_2\text{H}_5\text{OH}\) — combined atoms: 1 P, 5 Cl, 2 C, 6 H, 1 O.

Known products: \(\text{C}_2\text{H}_5\text{Cl}\) (2 C, 5 H, 1 Cl) + \(\text{HCl}\) (1 H, 1 Cl). Subtracting: remaining atoms in X = 1 P, 3 Cl, 0 C, 0 H, 1 O — i.e., \(\text{POCl}_3\).

\(M_r(\text{POCl}_3) = 31 + 16 + 3(35.5) = 153.5\). Mass of chlorine in X = \(3 \times 35.5 = 106.5\).

%Cl = \(\frac{106.5}{153.5} \times 100\% = 69.4\%\) → B.

Get the Sec 3 / Sec 4 recording package

Every O Level Pure Chemistry TYS MCQ solution video, plus full Sec 3 / Sec 4 topic recordings.