The concentration of a hydrochloric acid solution is $0.5 \mathrm{~mol} / \mathrm{dm}^{3}$.
How many moles of hydrochloric acid are present in $25 \mathrm{~cm}^{3}$ of this solution?

A sample of an iron oxide contains 50.4 g of iron and 21.6 g of oxygen.
What is the empirical formula of the iron oxide?