Question

Ammonium molybdate is used to test the radical:
(A)- $P{O_4}^{3-}$
(B)- $As{O_4}^{3-}$
(C)- $C{u}^{2+}$
(D)- $A{g}^{+}$

Answer 1

Hint: The molybdate blue reaction is the most common reaction used for the determination of radicals of 14 and 15 groups. The molybdate reaction occurs in two stages, the first stage involves the formation of a Keggin ion around the analyte anion and the second stage involves the reduction of heteropoly acid to a deeply blue-colored product.

Complete answer:
-The presence of phosphate ions can be tested by first acidifying the salt with concentrated nitric acid and adding a little amount of ammonium molybdate. This will give a bright yellow precipitate layer of ammonium phosphomolybdate.
$H_{3}P{O}_4+12{(N{H}_4)}_{2}Mo{O}_4+21HN{O}_3\to {(N{H}_4)}_{3}P{O}_4.12Mo{O}_3\downarrow +21N{H}_{4}N{O}_3+12H_{2}O$

-The same procedure is followed for testing the presence of arsenic radicles.
$H_{3}As{O}_4+12{(N{H}_4)}_{2}Mo{O}_4+21HN{O}_3\to {(N{H}_4)}_{3}As{O}_4.12Mo{O}_3\downarrow +21N{H}_{4}N{O}_3+12H_{2}O$

-For testing the presence of copper cations can be done by adding a few drops of sodium hydroxide solution to the aqueous solution of the salt. The formation of a blue precipitate is indicative of the copper radicles.
$$C{u}^{2+}+2O{H}^{-}\to Cu{(OH)}_2$$

-The presence of $A{g}^{+}$ ions can be tested by making the original solution of the salt and then diluting it with hydrochloric acid. A white precipitate is formed which is soluble with ammonia. The confirmation test of $A{g}^{+}$ ions can be done by adding potassium chromate to the original solution. A brick-red colored precipitate will indicate the presence of $A{g}^{+}$ ions.
$$A{g}^{+}+K_{2}Cr{O}_4\to A{g}_{2}Cr{O}_4\downarrow +2K^{+}$$

So, the correct answer is Ammonium molybdate is used to test the presence of $P{O_4}^{3-}$ and $As{O_4}^{3-}$ radicles, i.e option A and B.

Note:
Ammonium molybdate is found in five different forms- Ammonium orthomolybdate ${(N{H}_4)}_{2}Mo{O}_4$ ; Ammonium heptamolybdate ${(N{H}_4)}_{6}M{o}_7{O}_{24}$ ; Ammonium phosphomolybdate ${(N{H}_4)}_{3}PM{o}_{12}{O}_{40}$ ; Ammonium dimolybdate ${(N{H}_4)}_{2}M{o}_2{O}_7$ and Ammonium tetrathiomolybdate ${(N{H}_4)}_{2}Mo{S}_4$. Ammonium orthomolybdate is used as a corrosion inhibitor. Ammonium tetrathiomolybdate is used for the analysis of chloride in a solution. Ammonium heptamolybdate is used as a reagent for testing the presence of phosphates, silicates, arsenates, and lead in the aqueous solutions.