JEE Main · 2021 · 24 Feb 2021 · Shift 2 · Some Basic Concepts of Chemistry

Complete combustion of the hydrocarbon follows

At the same temperature and pressure equal volumes of gases contain equal numbers of molecules, so the volume ratios equal the mole ratios of the equation, and water is not counted because it condenses. One volume of hydrocarbon therefore needs $\left(x+\frac{y}{4}\right)$ volumes of oxygen and yields $x$ volumes of carbon dioxide, giving $x=4$ and $x+\frac{y}{4}=6$. Substituting $x=4$ leaves $\frac{y}{4}=2$, so $y=8$.