Percentage yield and purity

0.25 mol of an impure sample of magnesium carbonate, MgCO$_3$, reacts completely with excess dilute hydrochloric acid. The equation is:
$$\text{MgCO}_3 + 2\text{HCl} \rightarrow \text{MgCl}_2 + \text{H}_2\text{O} + \text{CO}_2$$
If 4.4 g of CO$_2$ is produced, what is the percentage purity of the MgCO$_3$ sample? ($M_r$ of CO$_2$ = 44)