What weight of potassium chlorate is required to prepare 89.6mL of oxygen measured over water at 21.0 degrees C and 743 millimeters of mercury?
Solve for n = number of moles oxygen in PV = nRT, then follow the steps (the process) in this link to calculate grams KClO3.
http://www.jiskha.com/science/chemistry/stoichiometry.html