When 15cm^3 of a gaseous hydrocarbon P, was burnt in 100cm^3 of oxygen, gaseous mixture occupied 70 cm^3 at room temperature and pressure. When the mixture was passed through potassium hydroxide, its volume decreased to 25 cm^3. a).What volume of the oxygen was used during the reaction? b).Determine the molecular formula of the hydrocarbon. (KCSE 2013 paper 1 question 23)