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The colour of ammonium phosphomolybdate is:
(A) Green
(B) yellow
(C) Blue
(D) Colourless

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Last updated date: 22nd Feb 2024
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IVSAT 2024
Answer
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Hint: Ammonium phosphomolybdate is an inorganic salt and it contains phosphomolybdate ion complexes. It is widely used for estimation of $PO_4^{ - 3}$ in soil and also in analytical estimation. The reaction involved in the preparation of ammonium phosphomolybdate is very important to solve this question.

Complete step by step solution:
Ammonium phosphomolybdate is an inorganic ammonium salt. The chemical formula of ammonium phosphomolybdate is ${(N{H_4})_3}.P{O_4}.12Mo{O_4}$.
Preparation of Ammonium phosphomolybdate:
1. The phosphate of metals such as calcium is heated with concentrated nitric acid to produce calcium nitrate and phosphoric acid.
\[C{a_2}{(P{O_4})_2} + 6HN{O_3} \to 3Ca{(N{O_3})_2} + 2{H_3}P{O_4}\]
2. Ammonium molybdate is heated in excess of concentrated nitric acid and phosphoric acid to yield ammonium phosphomolybdate.
 \[12{(N{H_4})_2}Mo{O_4} + 21HN{O_3} + {H_3}P{O_4} \to {(N{H_4})_3}P{O_4}.12Mo{O_3} + 12{H_2}O + 21N{H_4}N{O_3}\]
Ammonium molybdate [${(N{H_4})_3}Mo{O_4}$] is a white solid and the ammonium phosphomolybdate is obtained as a yellow precipitate. It is used for estimation of $PO_4^{ - 3}$ in a given analytical experiment and in soil.
Therefore, from the above discussion, it is evident that ammonium phosphomolybdate is yellow in colour. Thus Option B is the correct answer.
Additional information: The preparation of ammonium arsenomolybdate ${(N{H_4})_2}.As{O_4}.12Mo{O_3}$ is same as the preparation of ammonium phosphomolybdate but, there is a small change is that instead of ${H_3}P{O_4}$, arsenic acid (${H_3}As{O_4}$).
\[12{(N{H_4})_2}.Mo{O_4} + {H_3}As{O_4} + 21HN{O_3} \to {(N{H_4})_3}.As{O_4}.12Mo{O_3} + 12{H_2}O + 21N{H_4}N{O_3}\]
The ammonium arsenate phosphate also gives out the yellow precipitate. It is also used to estimate $AsO_4^{ - 3}$.

Hence, the answer is option (B).

Note: The chemical formula of ammonium phosphomolybdate is ${(N{H_4})_3}.P{O_4}.12Mo{O_4}$ whereas, the chemical formula of ammonium arsenomolybdate is ${(N{H_4})_2}.As{O_4}.12Mo{O_3}$. Ammonium phosphomolybdate is used for estimation of $PO_4^{ - 3}$ and ammonium arsenomolybdate is used for estimation of $AsO_4^{ - 3}$. Both Ammonium phosphomolybdate and ammonium arsenomolybdate are compounds of ammonium molybdate.