Question

What is its formula when $MgS{O_4}$ on reaction with $N{H_4}OH$ and $N{a_2}HP{O_4}$ forms a white crystalline precipitate?
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. $MgS{O_4}$

Answer 1

This question gives the knowledge about the precipitation reactions. Precipitation reactions are the reactions which occur when two soluble salts containing solutions combine to form an insoluble salt. That insoluble salt is called precipitate.

Complete step-by-step answer: When magnesium sulphate $MgS{O_4}$ reacts with disodium hydrogen phosphate and ammonium hydroxide, it leads to the formation of a white precipitate of magnesium ammonium phosphate.

The reaction is as follows:

$MgS{O_4} + N{H_4}OH + N{a_2}HP{O_4} \to Mg\left( {N{H_4}} \right)P{O_4} \downarrow + N{a_2}S{O_4} + {H_2}O$

In the above reaction Magnesium ammonium phosphate $Mg\left( {N{H_4}} \right)P{O_4}$ is precipitated out which is white in color.

Additional information: Precipitation reactions are the reactions which occur when two soluble salts containing solutions combine to form an insoluble salt. That insoluble salt is called precipitate. Precipitate reactions are very helpful in determining the presence of different ions in the solution. A precipitate is generally insoluble in the solution. Precipitation reactions are generally double displacement reactions.
The components which get precipitated during the reactions are generally reactants which can be present in any state solid, liquid or gas.
The most common example of precipitation reaction is the reaction between silver nitrate and potassium chloride which leads to the formation of a solid precipitate of silver chloride. The reaction is as follows:
$AgN{O_3} + KCl \to AgCl + KN{O_3}$
Other example of precipitation reaction is the reaction in which magnesium sulphate $MgS{O_4}$ reacts with disodium hydrogen phosphate and ammonium hydroxide, to lead the formation of a white precipitate of magnesium ammonium phosphate.

Hence, option $1$ is correct.

Note: Always remember that the precipitation reactions are the double displacement reactions. And also remember that precipitates are insoluble in the solution. These reactions are carried out under certain conditions.