A sample of $15.0\ \text{mL}$ of a gaseous hydrocarbon is mixed with $120.0\ \text{mL}$ of oxygen and burnt completely. After cooling to room temperature, the gas has a volume of $97.5\ \text{mL}$. After it is passed through aqueous KOH, the volume falls to $52.5\ \text{mL}$. All volumes are measured at the same temperature and pressure. What is the formula of the hydrocarbon?