Question

In a gravimetric determination of P, an aqueous Solution of dihydrogen phosphate ion H₂PO₄^- is treated with a mixture of ammonium and magnesium ions to precipitate magnesium ammonium phosphate, Mg(NH₄)PO₄ .6H₂O. This is heated and decomposed to magnesium pyrophosphate, Mg₂P₂O₇ which is weighed. A Solution of H₂PO₄^- yielded 1.054 g of Mg₂P₂O₇. What weight of NaH₂PO₄ was present originally?

Answer 1

Since P atoms are conserved, applying POAC for P atoms, 

moles of P in NaH₂PO₄ = moles of P in Mg₂P₂O₇

1 x moles of NaH₂PO₄ = 2 x moles of Mg₂P₂O₇

(1 mole of NaH₂PO₄ contains 1 mole of P and 1 mole of Mg₂P₂O₇ contains 2 moles of P)

Wt. of NaH₂PO₄ = 1.14 g.