The balanced chemical reaction would be expressed as follows:
2KClO3 = 2KCl + 3O2
To determine the mass of oxygen gas that is produced from a given amount of potassium chlorate, we use the initial amount of the reactant and use the relations from the reaction of the substances involved. However, from the problem, the amount of reactant is not given. We assume 1 g of potassium chlorate.
1 g KClO3 ( 1 mol / 122.55 g ) ( 3 mol O2 / 2 mol KClO3 ) ( 32 g O2 / 1 mol O2 ) = 0.3917 g of O2 produced.
If the given amount of the reactant is not 1 g, then you can change the value of the mass from the calculation and evaluate the new value of oxygen gas produced.
