So for hydrogen peroxide, the mass percent of hydrogen is the mass

of hydrogen in the compound divided by the total mass of the compound.

Now we know that the mass of one hydrogen is about

one gram per mole, but hydrogen peroxide has two atoms of hydrogen.

So the mass of hydrogen in hydrogen

peroxide is 2.0158 grams per mole, because there's

two hydrogens. We have to divide that by the molar mass

of the entire compound. Multiply that by 100%, and we see that the

mass percent of hydrogen in hydrogen peroxide is 5.926%.

The calculation for oxygen is really similar.

Again, there's two oxygens in the compound.

So, there would be 31.998 grams per mole of oxygen in hydrogen peroxide.

We divide that by the total mass of the compound, multiply it by 100%,

and get the mass percent of oxygen in hydrogen peroxide being about 94%.

We can check our work by adding the mass percent of all the elements up.

And they should equal 100%.