153.8 mg of oxygen in 324.0 mg of saltpeter
Since saltpeter has only potassium, nitrogen, and oxygen in it and since we've been told the amount of potassium and nitrogen in it, the amount of oxygen will be the total mass of saltpeter minus the known masses of potassium and nitrogen. So
100.00 g - 38.67 g - 13.86 g = 47.47 g
So now we know that 100.00 g of saltpeter has 47.47 g of oxygen. Just divide the mass of oxygen by the total mass of saltpeter and we can determine that 47.47% of the mass of saltpeter is oxygen.
So to determine the mass of oxygen in 324.0 mg of saltpeter, just multiply by 47.47%
324.0 mg * 47.47% = 324.0 mg * 0.4747 = 153.8 mg
