The formula of the salt in which the mass fraction of the metal is the largest? 1) FeCl32) MgCl23) CaCl2

Mr (FeCl3) = Ar (Fe) * N (Fe) + Ar (Cl) * N (Cl) = 56 * 1 + 35.5 * 3 = 162.5;
ω (Fe in FeCl3) = N (Fe) * Ar (Fe) * 100% / Mr (FeCl3) = 1 * 56 * 100% / 162.5 = 34.46%;

Mr (MgCl2) = Ar (Mg) * N (Mg) + Ar (Cl) * N (Cl) = 24 * 1 + 35.5 * 2 = 95;
ω (Mg in MgCl2) = N (Mg) * Ar (Mg) * 100% / Mr (MgCl2) = 1 * 24 * 100% / 95 = 25.26%;

Mr (CaCl2) = Ar (Ca) * N (Ca) + Ar (Cl) * N (Cl) = 40 * 1 + 35.5 * 2 = 111;
ω (Ca in CaCl2) = N (Ca) * Ar (Ca) * 100% / Mr (CaCl2) = 1 * 40 * 100% / 111 = 36.04%.

Answer: option 3).