A 5.5 g sample of a substance contains only carbon and oxygen. Carbon makes up 35% of the mass of the substance. The rest is made of oxygen. You are asked to determine the mass of oxygen in the sample.

Mass percentage is one way of representing the concentration of an element in a compound or a component in a mixture. Mass percentage is calculated as the mass of a component divided by the total mass of the mixture. We calculate as follows:

Percent O2 = 100 - 35 = 65%

Mass of oxygen = 5.5 x 0.65 = 3.575 g O2