The mass fraction of oxygen is the largest in the compound?

The mass fraction of oxygen in the compound will be greater where the molar masses of other elements included in the compound are smaller. The lightest element is hydrogen. Compounds of oxygen with hydrogen: H20 water and H202 – hydrogen peroxide.

There are 2 H atoms per 1 O atom in water, and 1 H atom in hydrogen peroxide. Therefore, the mass fraction of oxygen in H202 is higher.

w (O) H2O = M (O) / M (H2O) = 16/18 = 0.89; w (O) H2O = 89%;

w (O) H2O2 = M (O2) / M (H2O2) = 32/34 = 0.94; w (O) H2O2 = 94%.

Answer: H2O2.