The problem this causes in in determining the mass-to-mass ratio (the gravimetric ratio) between the salt and the elemental forms of As. I'll use Na2HAsO4.7H2O in the calculation, but you can follow the same procedure with any of the salt formulae. 1 mole of Na2HAsO4.7H2O: contains 1 mole of As Set up the gravimetric factor using this molar relationship. (1 mole of Na2HAsO4.7H2O)/(1 mole of As) = ((1 mole of Na2HAsO4.7H2O)*(312.01 g/mol))/((1 mole of As)*(74.92 g/mol)) = (312.01 g Na2HAsO4.7H2O)/(74.92 g As) = (4.16 g Na2HAsO4.7H2O)/(1 g As)