How many moles of oxygen will be produced by 0.66 moles of potassium chlorate?

Potassium chloride has the formula KClO3. When heated, it undergoes decomposition to produce oxygen gas according to the following equation:

For each mole of potatassium chlorate that reacts one mole of oxygen gas is produced. This tells us that 0.66 moles of KClO3 will produce 0.99 moles of O2:

0.66 moles KClO3 x (3 mole O2/2 mole KClO3) = 0.99 moles O2

The moles of a given quantity is multipled by the mole ratio of unknown to given, which is the ratio of coefficients from the balanced equation. In this case the coefficients are 2 and 3. 

Potassium chlorate can cause other substances to spontaneously combust, especially if it's heated. The oxygen produced creates a more flammable environment than the atmosphere alone, which is about 20% oxygen.