# Sulphur burns in air to form sulphur dioxide: S + O_2 -> SO_2. What is the mass of sulphur that must be burned to produce 64g of sulphur dioxide?

About $32 \cdot g$.
${\underbrace{S \left(s\right) + {O}_{2} \left(g\right) \rightarrow S {O}_{2} \left(g\right)}}_{\text{64 g of reactant gives 64 g of product}}$
And given molar masses, the reaction EXPLICITLY states that $32 \cdot g$ of sulfur is oxidized by $32 \cdot g$ to give $64 \cdot g$ of sulfur dioxide - these are all the equivalent masses of the reactants and products, which contain stoichiometric quantities of reactant and product.