Mr ( Cl2O7 ) = Ar ( Cl ) + Ar ( O ) = 35 * 2 + 16 * 7 = 70 + 112 = 182
Mr ( Na3PO4 ) = Ar ( Na ) + Ar ( P ) + Ar ( O ) = 23 * 3 + 31 + 16 * 4 = 69 + 31 + 64 = 164
Mr ( Al ( OH )3 ) = Ar ( Al ) + Ar ( O ) + Ar ( H ) = 27 + 16 * 3 + 1 * 3 = 27 + 48 + 3 = 78