A compound weighing 115 g contains 68.3 g of carbon, 13.2 g of hydrogen, 15.9 g of nitrogen, and some amount of oxygen. What is the mass percent of oxygen in the compound?
The balance, the mass that is not accounted for by carbon, hydrogen, and nitrogen is taken to be oxygen.
An analyst will typically report
Why? Because it is fairly difficult to measure oxygen (especially as the atmosphere is some 30% dioxygen).
Given this, the mass of oxygen present in your sample is: