Molar gas volume

An impure sample of magnesium carbonate has a mass of 2.50 g. It is reacted with excess hydrochloric acid and 560 cm³ of carbon dioxide is collected 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 of gas occupies 24 000 cm³ at r.t.p.; $M_r$ of $MgCO_3 = 84$.)