The correct chemical formula of ammonium phosphate is:
a.) $N_{2} H_{6} P O_{4}$
b.) $(N H_{3})_{3} P O_{4}$
c.) $(N H_{4})_{3} P O_{4}$
d.) $(N H_{4})_{2} P O_{4}$

Answer

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Hint: The valency of ammonium ion is +1 while the valency of phosphate ion is -3. They combine in an appropriate ratio to produce ammonium phosphate salt. It is formed by treating phosphoric acid with ammonia solution.

Complete step by step answer:
Ammonium phosphate is a salt of ammonia and orthophosphoric acid. Ammonium phosphate is highly unstable. Three moles of ammonium ion combine with one mole of phosphate ion to form one mole of the salt ammonium phosphate. The molecular formula of ammonium phosphate is

(N H_{4})_{3} P O_{4}

. Because of its instability, it is elusive and of no commercial value. However, a related double salt

(N H_{4})_{3} P O_{4} . (N H_{4})_{2} H P O_{4}

is also recognized but it is also unstable for practical use. Both tri-ammonium salts evolve ammonia. In contrast to the unstable nature of the tri-ammonium salts, the diammonium phosphate

(N H_{4})_{2} H P O_{4}

and the monoammonium salt

(N H_{4}) H_{2} P O_{4}

are stable salts that are commonly used as fertilizers to provide plants with fixed nitrogen and phosphorus.
Ammonium phosphate is prepared in laboratories by treating 85% phosphoric acid with 30% ammonia solution. It is a colourless, crystalline solid which has the odour of ammonia and is readily soluble in water. The chemical reaction for the same is:

H_{3} P O_{4} + 3 N H_{3} \to (N H_{4})_{3} P O_{4}

This salt converts into diammonium hydrogen phosphate

(N H_{4})_{2} H P O_{4}

.
Hence, the correct answer is (C).

Note: Remember that to form any known compound, we should know the oxidation states of their constituents. The constituents combine in appropriate ratios to give the desired molecule.

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