1) Mr(Fe(OH)₃) = Ar(Fe) + Ar(O)*3 + Ar(H)*3 = 56 + 16 * 3 + 1 * 3 = 51 + 56 = 107
2) Mr(K₂CO₃) = Ar(K) *2 + Ar(C) + Ar(O)*3 = 39 * 2 + 12 + 16 * 3 = 90 + 48 = 138
3) Mr(CuCl₂) = Ar(Cu) + Ar(Cl)*2 = 64 + 35 * 2 = 64 + 70 = 134
4) Mr(K₂SO₄) = Ar(K)*2 + Ar(S) + Ar(O)*4 = 39 * 2 + 32 + 16 * 4 = 110 + 48 = 158
