M(C3H8) = 44; C = 12 * 3 / 44 * 100% = 82% ; H = 1 * 8 / 44 * 100% = 18% M(Cu2S) = 160; Cu = 64 * 2 /160 * 100% = 80% ; S = 32 * 1 / 160 * 100% = 20% M(NH3) = 17; N = 14 * 1 /17 * 100% = 82% ; H = 1 * 3 / 17 * 100% = 18% M(MgO) = 40; Mg = 24 * 1 /40 * 100% = 60% ; O = 16 * 1 / 40 * 100% = 40% В некоторых случаях массовая доля примерна