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 magnesiumammonium 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 1.54 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 )

In a gravimetric determination of P, an aqueous solution of dihydrogen

1.	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 magnesiumammonium 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 1.54 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 )
Answer» Solution :`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 atomsareconserved,applyingPOAC forPatoms, moles of P in`NaH_(2) PO_(4) = ` moles of P in `Mg_(2) P_(2) O_(7)` `1 xx` molesof `NaH_(2) PO_(4) = 2 xx` moles of `Mg_(2) P_(2) O_(7)` (`:.` 1 moles of `NaH_(2) PO_(4)` contains 1 moleof P and 1 mole of `Mg_(2) P_(2) O_(7)`contains 2 molesof P) `(wt .of Na H_(2) PO_(4))/( mol . wt. of Na H_(2) PO_(4)) = 2 xx (wt. ofnMg_(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 studentsshouldnote that if in anyreaction, a particular ATOM is conserved, theprincipleof atom conservation(POAC) with RESPECT to thatatomcan be appliedregardlessof thenumberof stepsof the k reactionand theirsequence .]