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

 2 KClO3(s) → 3 O2(g) + 2 KCl(s) 

Note: MnO2 (Manganese Dioxide) is not part of the reaction. A catalyst lowers the activation energy and increases both forward and reverse reactions at equal rates. 

molar mass of KClO3 = 122.5
Moles of KClO3 =  3.45 / 122.55 = 0.028

Moles of O2 produce =

= 0.042 moles

molar mass of O2 = 32

so, mass of O2 = 32 x 0.042  = 1.35 g