A known mass of the organic compound is heated with fuming nitric acid, when the phosphorus present in the organic compound is oxidised to phosphoric acid. The phosphoric acid obtained is treated with magnesia mixture (a mixture of `MgCl_2`, `NH_4Cl` and `NH_4OH`) to give a precipitate of `Mg(NH_4)PO_4` which on ignition gives `Mg_2P_2O_7`.

Moles of `Mg_2P_2O_7 = (text(wt. of)quad Mg_2P_2O_7)/(text(Molar mass of)quad Mg_2P_2O_7)`

Moles of `P` in `Mg_2P_2O_7 = (2 xx text(wt. of)quad Mg_2P_2O_7)/(text(Molar mass of)quad Mg_2P_2O_7)`

= Moles of `P` in organic compound

Mass of phosphorous in organic compound `= (2 xx text(wt. of)quad Mg_2P_2O_7 )/(text(Molar mass of)quad Mg_2P_2O_7 ) xx 31`

Percentage of phosphorus `= (2 xx 31 xx text(wt. of)quad Mg_2P_2O_7)/(text(Molar mass of) quad Mg_2P_2O_7 xx text(weight of organic compound)) xx 100`

`= (62 xx text(weight of)quad Mg_2P_2O_7 )/(222 xx text(weight of organic compound)) xx 100`