Calculate the mass of o2 produced if 3.450 g potassium chlorate is completely decomposed by heating in presence of a catalyst (manganese dioxide)

The reaction is:

2 KClO3(s) → 3 O2(g) + 2 KCl(s) 

A catalyst simply lowers the activation energy so MnO2 is not part of the overall reaction.

By stoichiometry: 

3.45 g KClO3 x 1 mol/ 122.55g KClO3 x 3 mol O2/ 2 mol KClO3 x 31.99g/ 1 mol O2 = 331.096/ 245.1 = 1.35 grams O2 produced 

1.35 grams O2

Must be negative.substance increases