Question

Sodium chlorate (NaClO3(s)) dissociates, during moderate heating to produce solid sodium chloride (NaCl(s)) and gaseous oxygen (O2 (g)). A 3.4671 g of impure sodium chlorate was heated until the evolution of oxygen stopped. The mass of sample residue was subsequently found to be 2.9073 g. What wt% of the original impure sample was really sodium chlorate? Assume that no gases will be evolved from the impurities.