A student heats 30.0 g of a mixture of potassium chlorate and potassium chloride with a little…

Step 1. Moles of oxygen. $\dfrac{6.72\ \text{L}}{22.4\ \text{L/mol}} = 0.300\ \text{mol}$.

Step 2. Moles of the chlorate. $2\text{KClO}_{3}\ \rightarrow \ 2\text{KCl}\ +\ 3\text{O}_{2}$. So $0.300 \times \dfrac{2}{3} = 0.200\ \text{mol}$.

Step 3. Mass. $0.200\ \text{mol} \times 122.5\ \text{g/mol} = 24.5\ \text{g}$.

Step 4. The percentage. $\dfrac{24.5\ \text{g}}{30.0\ \text{g}} \times 100 = 81.7\ \%$.

Step 5. The solid left. The oxygen has a mass of $0.300 \times 32.0 = 9.60\ \text{g}$. The solid is $30.0 - 9.60 = 20.4\ \text{g}$. It is all potassium chloride.