Burning non-metals

Carbon, sulphur and hydrogen burn in oxygen: $\text{C} \rightarrow \text{CO}_2$, $\text{S} \rightarrow \text{SO}_2$, $\text{H}_2 \rightarrow \text{H}_2\text{O}$. You will make the program find the mass of oxide from $8\ \text{g}$ of each, the oxygen that each uses, and the element that needs the most oxygen.

The idea. For each element, a fixed mass gives a fixed mass of oxide: $12\ \text{g}$ of carbon gives $44\ \text{g}$ of $\text{CO}_2$, $32\ \text{g}$ of sulphur gives $64\ \text{g}$ of $\text{SO}_2$, $2\ \text{g}$ of hydrogen gives $18\ \text{g}$ of water. The oxygen used is the oxide minus the element.

Your tasks.

1. In oxide_mass, return the oxide from m grams, given the pair (a, b): a grams give b grams.
2. Fill oxygen with the oxygen used by $8\ \text{g}$ of each element.
3. Find most_oxygen, the element that uses the most oxygen.