At elevated temperatures, solid sodium chlorate (NaClO₃) decomposes to produce sodium chloride, NaCl, and O₂ gas. A 0.8765 g sample of impure sodium chlorate was heated until the production of oxygen ceased. The oxygen gas was collected over water and occupied a volume of 57.2 mL at 23.0 °C and 734 Torr. Calculate the mass percentage of NaClO₃ in the original sample. Assume that none of the impurities produce oxygen on heating. The vapor pressure of water is 21.07 Torr at 23.0 °C.