2023 H2 Chem P1 Q23 — Mole Solution

The answer is A (1, 2 and 3).
\(\text{C}_x\text{H}_y + (x + \frac{y}{4})\text{O}_2 \rightarrow x\text{CO}_2 + \frac{y}{2}\text{H}_2\text{O}\)
Volumes of gases at the same temperature and pressure are in the mole ratio, so X : \(\text{O}_2\) : \(\text{CO}_2\) = 10 : 75 : 50 = 1 : 7.5 : 5.
So \(x = 5\) and \(5 + \frac{y}{4} = 7.5\), giving \(y = 10\). X is \(\text{C}_5\text{H}_{10}\), which has one ring or one C=C bond.
• Statement 1, correct. X can be a cycloalkane, e.g. cyclopentane, \(\text{C}_5\text{H}_{10}\).
• Statement 2, correct. With five carbons, X can have a branched chain, e.g. 3-methylbut-1-ene, \(\text{CH}_2\text{=CHCH(CH}_3)_2\).
• Statement 3, correct. X can be an alkene, e.g. pent-2-ene, \(\text{CH}_3\text{CH=CHCH}_2\text{CH}_3\), which decolourises aqueous bromine in the dark by electrophilic addition.
• B is wrong: it leaves out statement 3, but an alkene isomer of \(\text{C}_5\text{H}_{10}\) decolourises aqueous bromine.
• C is wrong: it leaves out statement 2, but branched isomers such as 3-methylbut-1-ene exist.
• D is wrong: it leaves out statement 1, but cyclopentane has the formula \(\text{C}_5\text{H}_{10}\).