Chemical formula of hydrogen peroxide is H2O2. Atomic mass of O is 16 u. So mass of 2 O = 2 *16 = 32 Molecular mass of H2O2 = 2*1 + 2*16 = 34 Mass percent of O = Total mass of O in compound/ Total mass of compound * 100 = 32/34 *100 = 94.12. read more