Can you show me how to do this?

2KClO3 -> 2KCl + 3O2
If 5.0(g) of KClO3 is decomposed , what volume of O2 is produced at STP?

5 g KClO3 = ?? mols.
??mols KClO3 will produce how many mols O2? Use the coefficients in the equation to do this conversion.

1 mol O2 occupies 22.4 L at STP. Convert mols you have to Liters.

Post any work if you get stuck and need further assistance. Please be specific about what you don't understand.