# How would you calculate the percentage by mass of oxygen in a compound?

"%Oxygen"="Mass of oxygen"/"Formula mass of compound"xx100%
Let's try this with $\text{potassium permangante, } K M n {O}_{4}$, whose formula mass is $158.03 \cdot g \cdot m o {l}^{-} 1$.
"% oxygen "= (4xx15.999*g*mol^-1)/(158.03*g*mol^-1)xx100%~=40%