Calculating m percentages?

At elevated temperatures, sodium chlorate decomposes to produce sodium chloride and oxygen gas. A 0.8735 g sample of impure sodium chlorate was heated until the production of oxygen gas ceased. The oxygen gas collected over water occupied 57.2 mL at a temperature of 22�C and a pressure of 750. torr. Calculate the m percent of NaClO3 in the original sample. (At 22�C the vapor pressure of water is 19.8 torr.)