A) \[{{H}_{3}}P{{O}_{4}}\]
B) \[{{P}_{2}}{{O}_{5}}\]
C) \[M{{g}_{2}}{{P}_{2}}{{O}_{7}}\]
D) \[M{{g}_{3}}{{(P{{O}_{4}})}_{2}}\]
Correct Answer: C
Solution :
A pre-weighed amount of organic compound is heated with concentrated nitric acid \[(HN{{O}_{3}})\]in a tube. . Phosphorus present in the compound gets converted into phosphoric acid. Now the content of tube are heated with magnesia mixture, i.e., \[MgS{{O}_{4}}+N{{H}_{4}}Cl+N{{H}_{4}}OH,\]so that the magnesium ammonium phosphate \[MgN{{H}_{4}}P{{O}_{4}}\]is formed which is collected, washed, dried and ignited to get magnesium pyrophosphate\[M{{g}_{2}}{{P}_{2}}{{O}_{7}},\]which is then weighed. \[\underset{\text{Phosphoric}\,\text{acid}}{\mathop{{{H}_{3}}P{{O}_{4}}}}\,\xrightarrow{\text{Magnesia}\,\text{mixture}}\,\underset{\begin{smallmatrix} \text{Magnesium}\,\text{ammonium} \\ \text{Phosphate}\,\text{(white}\,\text{ppt}\text{.)} \end{smallmatrix}}{\mathop{MgN{{H}_{4}}P{{O}_{4}}}}\,+3HCl\] \[2MgN{{H}_{4}}P{{O}_{4}}\xrightarrow{\Delta }M{{g}_{2}}{{P}_{2}}{{O}_{7}}\downarrow +2N{{H}_{3}}+{{H}_{2}}O\] From here knowing the weight of \[M{{g}_{2}}{{P}_{2}}{{O}_{7}},\]the percentage of phosphorus can be found out. Hence, the correct option is (c).
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