Explanation:

The balanced chemical equation for the decomposition of potassium chlorate is:

$$2KClO_3 \rightarrow 2KCl + 3O_2$$

This means that for every 2 moles of potassium chlorate that decompose, 3 moles of oxygen are produced. Therefore, 12 moles of potassium chlorate will produce:

$$12 \ \text{mol KClO}_3 \ \times \frac{3 \ \text{mol O}_2}{2 \ \text{mol KClO}_3} = 18 \ \text{mol O}_2$$