Question

\[\text{MgS}{{\text{O}}_{\text{4}}}\]on reaction with \[\text{N}{{\text{H}}_{4}}\text{OH and N}{{\text{a}}_{2}}\text{HP}{{\text{O}}_{\text{4}}}\] forms a white crystalline precipitate. What is its formula?
A) \[\text{Mg}\left( \text{N}{{\text{H}}_{4}} \right)\text{P}{{\text{O}}_{\text{4}}}\]
B) \[\text{M}{{\text{g}}_{3}}{{\left( \text{P}{{\text{O}}_{\text{4}}} \right)}_{2}}\]
C) \[\text{MgC}{{\text{l}}_{2}}\centerdot \text{MgS}{{\text{O}}_{\text{4}}}\]
D) \[\text{MgS}{{\text{O}}_{\text{4}}}\]

Answer 1

Hint: In the formula of precipitate, the atoms from \[\text{MgS}{{\text{O}}_{\text{4}}}\], \[\text{N}{{\text{H}}_{4}}\text{OH and N}{{\text{a}}_{2}}\text{HP}{{\text{O}}_{\text{4}}}\] will be present.

Complete step by step answer:
 \[\text{MgS}{{\text{O}}_{\text{4}}}\]on reaction with \[\text{N}{{\text{H}}_{4}}\text{OH and N}{{\text{a}}_{2}}\text{HP}{{\text{O}}_{\text{4}}}\] forms a white crystalline precipitate of magnesium ammonium phosphate. Other byproducts of the reaction are \[\text{N}{{\text{a}}_{2}}\text{S}{{\text{O}}_{4}}\text{ and }{{\text{H}}_{2}}\text{O}\]. Write the balanced chemical equation.
\[\begin{align}
  & \text{MgS}{{\text{O}}_{4}}\text{ + N}{{\text{H}}_{4}}\text{OH + N}{{\text{a}}_{2}}\text{HP}{{\text{O}}_{4}}\text{ }\to \text{Mg}\left( \text{N}{{\text{H}}_{4}} \right)\text{P}{{\text{O}}_{\text{4}}}\downarrow \text{ + N}{{\text{a}}_{2}}\text{S}{{\text{O}}_{4}}\text{ + }{{\text{H}}_{2}}\text{O } \\
 & \text{ White ppt } \\
\end{align}\]
In the above equation, sulphate ions, hydroxide ions and sodium ions are spectator ions. Write the net ionic equation by ignoring these spectator ions:
\[\begin{align}
  & \text{M}{{\text{g}}^{2+}}\text{ + N}{{\text{H}}_{3}}\text{ + HPO}_{4}^{2-}\text{ }\to \text{MgN}{{\text{H}}_{4}}\text{P}{{\text{O}}_{\text{4}}}\downarrow \\
 & \text{ White ppt } \\
\end{align}\]
In the above net ionic equation, magnesium cations combine with ammonia and monohydrogen phosphate anion, to form white precipitate of magnesium ammonium phosphate.
However, in the neutral medium, a white flocculent precipitate of magnesium hydrogen phosphate is obtained.
\[\begin{align}
  & \text{M}{{\text{g}}^{2+}}\text{ + HPO}_{4}^{2-}\text{ }\to \text{MgHP}{{\text{O}}_{\text{4}}}\downarrow \\
 & \text{ White ppt is obtained } \\
\end{align}\]
Hence, the option A) \[\text{Mg}\left( \text{N}{{\text{H}}_{4}} \right)\text{P}{{\text{O}}_{\text{4}}}\] is the correct option.

Note:
Do not give the formula of the precipitate based on the product in the neutral medium since a white flocculent precipitate of magnesium hydrogen phosphate is obtained when reacted in neutral medium