Calculate the mass fractions of oxygen in the compounds: H2O2, HgO, KMnO4, KCIO3

The mass fraction of an element in a substance is found by dividing the atomic mass of an element multiplied by its amount, by the total mass of the substance.
In the first case, we have hydrogen peroxide: w = M (O) * 2 / M (H2O2) = 16 * 2/34 = 0.941 = 94.1%
The second case is from mercury oxide: w = M (O) / M (HgO) = 16 / (217) = 0.074 = 7.4%
Third case from potassium permanganate: w = M (O) * 4 / M (KMnO4) = 16 * 4/158 = 0.405 = 40.5%
The fourth case of potassium chlorate: w = M (O) * 3 / M (KClO3) = 16 * 3 / 122.5 = 0.392 = 39.2%