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

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.

& \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{aligned}$$ In the above equation, sulphate ions, hydroxide ions and sodium ions are spectator ions. Write the net ionic equation by ignoring these spectator ions: $$\begin{aligned} & \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{aligned}$$ In the above net ionic equation, magnesium cations combine with ammonia and monohydrogen phosphate anion, to form white precipitate of magnesium ammonium phosphate. However, in the neutral medium, a white flocculent precipitate of magnesium hydrogen phosphate is obtained. $$\begin{aligned} & \text{M}{{\text{g}}^{2+}}\text{ + HPO}_{4}^{2-}\text{ }\to \text{MgHP}{{\text{O}}_{\text{4}}}\downarrow \\\ & \text{ White ppt is obtained } \\\ \end{aligned}$$ **Hence, the option A) $$\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