# A 230 g sample of a compound contains 136.6 g of carbon. 26.4 g of hydrogen, and 31.8 g of nitrogen. The rest is oxygen. What is the mass percent of oxygen in the compound?

$\frac{\left(230 - 136.6 - 26.4 - 31.8\right) \cdot g}{230 \cdot g}$ $\times$ 100% $=$ ??%
The mass of oxygen in the $230$ $g$ sample is simply that mass that is not $C$, $H$, or $N$. So I simply subtract from the starting mass and the balance must be oxygen.
In fact there are few ways that an analyst can measure oxygen. Typically, a sample containing $C , H , N , O$ will be presented, and the percentage $C$, $H$, $N$ will be reported. The percentage $O$ is the percentage not accounted for is assumed to be oxygen.