Analytical Chemistry Question

Question

The active ingredient in a zinc nutritional supplement is ZnSO4⋅7H2OZnSO4⋅7H2O (molar mass = 287.58 g/mol). Ten zinc tablets containing a mix of the active ingredient and an inactive filler were ground and mixed together thoroughly. The total mass of these 10 tablets was 11.39511.395 g. A 1.998 g sample of the finely ground tablets was dissolved in an acidic solution. The solution was neutralized with ammonia and subsequently warmed in a water bath. Addition of (NH4)3PO4 to the solution resulted in the precipitation of ZnNH4PO4 The precipitate was filtered and dried, then ignited to produce 0.220 g of Zn2P2O7 (molar mass = 304.76 g/mol). What is the mass percent of ZnSO4⋅7H2O in each tablet?

Answer 1

0.220 g Zn₂P₂O₇ x 1 mol Zn₂P₂O₇ / 304.76 g = 7.219x10^-4 moles Zn₂P₂O₇

7.219x10^-4 moles Zn₂P₂O₇ x 2 mol Zn / mol Zn₂P₂O₇ = 1.444x10^-3 moles Zn

1.444x10^-3 moles Zn x 1 mol ZnSO₄⋅7H₂O / mol Zn = 1.444x10^-3 moles ZnSO₄⋅7H₂O

Mass ZnSO₄⋅7H₂O = 1.444x10^-3 moles ZnSO₄⋅7H₂O x 287.58 g / mol = 0.4153 g ZnSO₄⋅7H₂O

Total mass Mass ZnSO₄⋅7H₂O = 0.4153 g ZnSO₄⋅7H₂O / 1.998 g x 11.395 g = 2.369 g ZnSO₄⋅7H₂O in original sample

% by mass ZnSO₄⋅7H₂O in original sample = 2.369 g / 11.395 g (x100%) = 20.79%