# When 5.0 g of tin reacts with hydrochloric acid, the mass of the products, tin chloride and hydrogen, totals 8.1 g. How many grams of hydrochloric acid were used?

The mass of hydrochloric acid that was used is $\text{3.1 g}$.
So, if the mass of the products totals $\text{8.1 g}$, then you know that the total mass of the reactants must be equal to the total mass of the products. If the mass of the reactant tin is $\text{5.0 g}$, then the mass of the reactant hydrochloric acid is the difference between the total mass of the products and the mass of the reactant tin, as long as the reaction took place in a closed system.
$\text{8.1 g"-"5.0 g"="3.1 g}$