`Mg` is precipitated and estimated gravimetrically as:

Options

A. `Mg (NH_(4))PO_(4)`
B. `Mg (NH_(4)PO_(4).6H_(2)O`
C. `Mg_(2)P_(2)O_(7)`
D. `Mg_(3)(PO_(4))_(2)`

Answer

C

Explanation

`Mg^(2+)+HPO_(4)^(2-)+NH_(4)^(o+)+overset(Θ)(OH)rarrMg(NH_(4))PO_(4)+H_(2)O`
A cold acid solution of magnesium salt is treated with an excess of diammonium hydrogen phosphate and then excess of ammonia solution is added to precipitate out magnesium ammonium phophate hexahydrate, `Mg(NH_(4))PO_(4).6H_(2)O`. this precipitate is then filtered, washed and ignited `(1000-1100^(@)C )` to magnesium pyrophosphate `(Mg_(2)P_(2)O_(7))` and weighed as such.
`2MgNH_(4)PO_(4)overset(1000-1100^(@)C)(rarr)Mg_(2)P_(2)O_(7)+2NH_(3)+H_(2)O`