Calculate the mass fraction of impurities contained in technical sodium nitrate, if heating 60 g of it yields

Calculate the mass fraction of impurities contained in technical sodium nitrate, if heating 60 g of it yields oxygen with a volume of 6.72 liters.

Let’s write the reaction equation: 2NaNO3 = 2NaNO2 + O2
Let us find the amount of oxygen substance: 6.72 / 22.4 = 0.3 (mol)
The amount of sodium nitrate substance is twice the amount of oxygen substance.
Let’s find the mass of sodium nitrate: 0.6 * 85 = 51 (g).
Find the mass of impurities: 60-51 = 9 (g)
Let’s calculate the mass fraction of impurities: 9/60 = 0.15 or 15%