The mole

An impure sample of magnesium carbonate has a mass of 2.50 g. It reacts completely with excess hydrochloric acid to produce 560 cm³ of carbon dioxide at room temperature and pressure.

$MgCO_3 + 2HCl \rightarrow MgCl_2 + CO_2 + H_2O$

What is the percentage purity of the magnesium carbonate? (1 mol gas at r.t.p. occupies 24 000 cm³.)