Write the chemical formulae for the following compounds and find the ratio by mass of the combining elements in each one of them.
$(N=14 \ u, H=1 \ u, C=12 \ u, O=16 \ u, S=32 \ u)$
$(a)$ Methane
$(b)$ Carbon dioxide
$(c)$ Hydrogen sulphide

(A) Methane: Formula is $CH_{4}$. The ratio by mass of $C:H$ is $12:(4 \times 1) = 12:4 = 3:1$.
$(b)$ Carbon dioxide: Formula is $CO_{2}$. The ratio by mass of $C:O$ is $12:(2 \times 16) = 12:32 = 3:8$.
$(c)$ Hydrogen sulphide: Formula is $H_{2}S$. The ratio by mass of $H:S$ is $(2 \times 1):32 = 2:32 = 1:16$.