In a gravimetric determination of P, an aqueous solution of dihydrogen phosphate ion H 2 PO 4 – is treated with a mixture of ammonium and magnesium ions to precipitate magnesium ammonium phosphate, Mg(NH 4 )PO 4 .6H 2 O. This is heated and decomposed to magnesium pyrophosphate, Mg 2 P 2 O 7 , which is weighted. A solution of H 2 PO 4 – yeilded 1.054 g of Mg 2 P 2 O 7 . What weight of NaH 2 PO 4 was present originally?
Text Solution
Verified by ExpertsA
NaH 2 PO 4 + Mg 2+ + NH 4 + 
Mg(NH 4 )PO 4 . 6H 2 O 
Mg 2 P 2 O 7
Since P atoms are conserved, applying POAC for P atoms,
moles of P in NaH 2 PO 4 = moles of P in Mg 2 P 2 O 7
1 × moles of NaH 2 PO 4 = 2 × moles of Mg 2 P 2 O 7
( 1 mole of NaH 2 PO 4 contains 1 mole of P and 1 mole of Mg 2 P 2 O 7 contains 2 moles of P)
= 2 × 
= 2 × 
.
Wt. of NaH 2 PO 4 = 1.14 g.
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