With 140 cm³ oxygen supplied, 40 cm³ unabsorbed gas relighting a splint, and 3 carbons per molecule, the hydrocarbon is:

1. The unabsorbed gas relights a glowing splint, so it is leftover oxygen: 40 cm³ unused. 2. Oxygen actually consumed is $140-40=100$ cm³ for 20 cm³ of hydrocarbon. 3. That ratio is 5, so $x+\dfrac{y}{4}=5$. 4. With $x=3$ from the carbon dioxide, $\dfrac{y}{4}=2$, giving $y=8$. 5. The molecular formula is therefore $C_3H_8$, propane. _Source: NECTA ACSEE 2024 Chemistry 132/1, Question 1: Aliphatic Hydrocarbons_