Question
How the phosphorus is analysed in organic compound? Explain.

Answer

For the estimation of phosphorus. When the quantity of organic compound is heated with fumed nitric acid, the phosphorus present in it gets oxidized into phosphoric acid $\left( H _3 PO _4\right)$. On adding ammonia and ammonium molybdate solution, a precipitate of ammonium phosphomolybdate $\left[\left( NH _4\right)_3 PO _4 \cdot 12 MoO _3\right]$ is obtained or by adding magnesia mixture to phosphoric acid it is precipitated in form of $MgNH _4 PO _4$, on the combustion of which $Mg _2 P _2 O _7$ is obtained.
Let the mass of organic compound $= m _1 g$ and the mass of ammonium phosphomolybdate is $m _1 g$. Molar mass of $\left( NH _4\right)_3 PO _4 \cdot 12 MoO _3=1877 gm$
Hence $\%$ of phosphorus $=\frac{31 \times m _1 \times 100}{1877 \times m } \%$
When phosphorus is analysed in form of $Mg _2 P _2 O _7$,
then $\%$ of phosphorus $=\frac{62 \times m_1 \times 100}{222 \times m}$
Here mass of $Mg _2 P _2 O _7=222 u$
Mass of organic compound $= m _1 g$
Mass of formed $Mg _2 P _2 O _7= m$, and mass of two phosphorus atoms in $Mg _2 P _2 O _7$ is 62 .

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