ACSEE (Form 6) Aliphatic Hydrocarbons — practice questions

1. Complete combustion gives carbon dioxide and water: $C_xH_y+O_2\rightarrow xCO_2+\dfrac{y}{2}H_2O$. 2. Balance the carbon first: $x$ carbons give $x$ molecules of $CO_2$, needing $x$ molecules of $O_2$. 3. Balance the hydrogen: $y$ hydrogens give $\dfrac{y}{2}$ molecules of water, which carry $\dfrac{y}{2}$ oxygen atoms. 4. Those $\dfrac{y}{2}$ atoms come from $\dfrac{y}{4}$ molecules of $O_2$. 5. Adding the two requirements gives $\left(x+\dfrac{y}{4}\right)$ moles of oxygen. _Source: NECTA ACSEE 2024 Chemistry 132/1, Question 1: Aliphatic Hydrocarbons_

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_