1. **State the problem:** Calculate the relative formula mass of methane (CH₄) and then find the percentage by mass of carbon in methane. 2. **Formula for relative formula mass:** The relative formula mass (Mr) is the sum of the relative atomic masses (Ar) of all atoms in the molecule. 3. **Calculate Mr of methane:** Methane has 1 carbon atom and 4 hydrogen atoms. $$Mr = (1 \times Ar_C) + (4 \times Ar_H)$$ Given: $Ar_C = 12$, $Ar_H = 1$ $$Mr = (1 \times 12) + (4 \times 1) = 12 + 4 = 16$$ 4. **Calculate percentage by mass of carbon:** Percentage by mass of an element = $\frac{\text{mass of element in formula}}{\text{relative formula mass}} \times 100$ For carbon in methane: $$\%C = \frac{12}{16} \times 100 = 75\%$$ **Final answers:** - Relative formula mass of methane = 16 - Percentage by mass of carbon in methane = 75%