$MgS{O_4}$ on reaction with $N{H_4}OH$ and $N{a_2}HP{O_4}$, forms a white crystalline precipitate. What is its formula $?$
A) $Mg\left( {N{H_4}} \right)P{O_4}$
B) $M{g_3}{\left( {P{O_4}} \right)_2}$
C) $MgC{l_2}.MgS{O_4}$
D) $MgS{O_4}$

Hint: The precipitation reaction involves the formation of salts which are insoluble in the solution of reaction where two solutions containing the soluble salts are reacted. The precipitation product usually involves the contents of the molecules which are reacted.

Complete step by step answer:
1) The precipitate is usually formed and gets settled at the bottom of the solution. This type of reaction involves the formation of salts that are insoluble in reaction solution when two solutions having soluble salts are reacted.
2) A precipitation reaction shows the presence of all the various ions present in that reaction solution. The calculation of the solubility table is performed to explain the precipitation reactions. The precipitation reaction usually involves the reaction of two solutions that have different salts that have a cation or an anion pair in the formation combined solution which leads to the formation of an insoluble salt in the reaction mixture. This insoluble salt formed later precipitates at the bottom of the solution.
3) The given reaction will proceed as follows:
$MgS{O_4} + N{a_2}HP{O_4} + N{H_4}OH \to Mg\left( {N{H_4}} \right)P{O_4} \downarrow + N{a_2}S{O_4} + {H_2}O$
In the above reaction when Magnesium sulfate is reacted with Sodium hydrogen phosphate and Ammonium hydroxide a white crystalline precipitate of Magnesium ammonium phosphate is formed which is denoted with a downward arrow along with anhydrous sodium sulfate in an aqueous medium.
4) Therefore, the white crystalline precipitate $Mg\left( {N{H_4}} \right)P{O_4}$ is formed in the reaction which shows option A as a correct choice.

Hence, option A is marked as the correct choice.

Note:
The formation of precipitate in a solution can be guessed by a concept of ‘like dissolves like’ which means polar salts dissolve actively only in polar solvents and the same applies for non-polar salts also. The solubility of a particular substance in a particular solvent can be measured by the amount of concentration of that substance in that saturated solution.