How many grams of potassium chlorate decompose to potassium chloride?

N = pV / RT = 0.914 x 0.565 L / 0.08206 x 401 K =0.0157 mol
the ratio between KClO3 and O2 is 2 : 3

moles KClO3 =0.0157 x 2 / 3 = 0.0105 mol

Mass KClO3 = 0.0105 mol x 122.553 g/mol =1.29 g

You have to find out how many moles of O2 you have at the pressure and temperature. Use PV=nRT then calculate grams from moles of KC103