How much oxygen will we get from 120 grams of KClO3?

Answer 1

none, unless you decompose it.
With this equation
2KClO3>>>2KCl + 3O2
for each mole of KClO3, you get 1.5 moles O2.

you started with molesKClO3 as 120/molmassKClO3

multiply that by 1.5 to get moles O2
multipy then by 32 to get grams of O2