Potassium chlorate is KClO3 and the decomposition looks like this...
2KClO3 ==> 2KCl + 3O2
4.50 g KClO3 x 1 mole/122.55 g = 0.0367 moles
0.0367 moles KClO3 x 3 moles O2/2 moles KClO3 = 0.0245 moles O2 (see mo
le ratio in balanced equation)
mass of O2 = 0.0245 moles O2 x 32 g/mole = 0.784 g O2 formed