At hihg temperatures Sodium chlorate decomposes to form Sodium Chloride and Oxygen gas. A 0.8765 gram sample of sodium chlorate was heated until the reaction ended when Oxygen gas production was ceased. The Oxygen gas collected over water occupied 57.2 ml at temperature of 295 Kelvin and pressure of 734 torr. Calculate the mass percent of Sodium chlorate in the original sample. (At 22 degrees Celsius the vapor pressure of water is 19.8)

First I tried to make a chemical reaction equation.
2NaClO3=>2NaCl + 3O2.