20 cm³ of a hydrocarbon burnt in excess oxygen leaves 100 cm³, of which KOH absorbs 60 cm³. The number of carbon atoms is:

1. Concentrated potassium hydroxide absorbs carbon dioxide, so the 60 cm³ removed is the $CO_2$ produced. 2. At the same temperature and pressure, gas volumes are in the same ratio as moles. 3. So 20 cm³ of hydrocarbon produced 60 cm³ of carbon dioxide. 4. That is a ratio of 1 to 3, meaning each molecule yields 3 molecules of $CO_2$. 5. Each carbon atom becomes one $CO_2$, so the hydrocarbon has 3 carbon atoms. _Source: NECTA ACSEE 2024 Chemistry 132/1, Question 1: Aliphatic Hydrocarbons_