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 weighed. A solution of `H_(2)PO_(4)^(-)` yielded `1.054 g of Mg_(2)P_(2)O_(7)`. What weight of `NaH_(2)PO_(4)` was present originally? `(Na=23, H=1, P= 31, O=16,Mg=24)`

Explanation

`Na H_(2) PO_(4) + Mg^(2+) + NH_(4)^(+) to Mg (NH_(4)) PO_(4). 6H_(2) O overset("heated") to 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 xx` moles of `NaH_(2) PO_(4) = 2 xx` moles of `Mg_(2) P_(2) O_(7)`
(`:.` 1 moles 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)
`(wt . of Na H_(2) PO_(4))/( mol . wt. of Na H_(2) PO_(4)) = 2 xx (wt. ofn Mg_(2) P_(2) O_(7))/( mol . wt . of Mg_(2) P_(2) O_(7))`
`(wt . of Na H_(2) PO_(4))/( 120 ) = xx (1.054)/( 222)`
Wt. of `NaH_(2) PO_(4) = 1.14`
[Note : in Ex . 5 and Ex. 6 the students should note that if in any reaction, a particular atom is conserved, the principle of atom conservation (POAC) with respect to that atom can be applied regardless of the number of steps of the k reaction and their sequence .]