Question

${\mathrm{MgSO}}_4$ on reaction with ${\mathrm{NH}}_4\mathrm{OH}$ and ${\mathrm{Na}}_2{\mathrm{HPO}}_4$ forms a white crystalline precipitate. What is its formula? (a) $\mathrm{Mg}\left({\mathrm{NH}}_4\right){\mathrm{PO}}_4$ (b) ${\mathrm{Mg}}_3{\left({\mathrm{PO}}_4\right)}_2$ (c) ${\mathrm{MgCl}}_2\cdot {\mathrm{MgSO}}_4$ (d) ${\mathrm{MgSO}}_4$

Short answer

The formula of the precipitate is $\mathrm{Mg}\left({\mathrm{NH}}_4\right){\mathrm{PO}}_4$.

Step-by-step solution

Understand Reaction Components

Identify the reactants involved. We have ${\mathrm{MgSO}}_4$, ${\mathrm{NH}}_4\mathrm{OH}$, and ${\mathrm{Na}}_2{\mathrm{HPO}}_4$. ${\mathrm{MgSO}}_4$ provides the ${\mathrm{Mg}}^{2+}$ ions. ${\mathrm{NH}}_4\mathrm{OH}$ provides the ${\mathrm{NH}}_4^{+}$ ions, and ${\mathrm{Na}}_2{\mathrm{HPO}}_4$ dissociates into ${\mathrm{PO}}_4^{3-}$ ions.

Predict Product Formation

Examine potential interactions between the ions. The ${\mathrm{Mg}}^{2+}$ ions can combine with the ${\mathrm{PO}}_4^{3-}$ ions to possibly form $\mathrm{Mg}\left({\mathrm{NH}}_4\right){\mathrm{PO}}_4$, as each magnesium ion can pair with a phosphate group taking into account the ammonium presence.

Balance Chemical Equation

For forming $\mathrm{Mg}\left({\mathrm{NH}}_4\right){\mathrm{PO}}_4$, we should balance both charge and atoms. We need one ${\mathrm{Mg}}^{2+}$, one ${\mathrm{NH}}_4^{+}$, and one ${\mathrm{PO}}_4^{3-}$ to neutralize charges and maintain stoichiometry, forming $\mathrm{Mg}\left({\mathrm{NH}}_4\right){\mathrm{PO}}_4$ as the precipitate.

Identify the Precipitate

From the balanced combination, the precipitate formed is $\mathrm{Mg}\left({\mathrm{NH}}_4\right){\mathrm{PO}}_4$, which is a white crystalline substance. Verify if this matches the description in the question. It corresponds to option (a).

Key concepts

Ionic Equations

When we talk about ionic equations, we are often referring to a way to express chemical reactions by focusing on the ions involved in the reaction. This is especially useful in reactions taking place in aqueous solutions where ions are the principal players. An ionic equation aims to simplify the reaction by displaying only the ions and molecules that participate directly in the chemical change. It is important to distinguish between complete ionic equations and net ionic equations.

- **Complete Ionic Equations**: Including all the ions present as reactants and products in the equation. They show all the details, including spectator ions which do not participate in the reaction itself.

- **Net Ionic Equations**: These show only the ions and molecules directly involved in forming the reaction's products. Spectator ions are excluded for clarity and simplicity.

For example, in the reaction where magnesium sulfate (${\mathrm{MgSO}}_4$) reacts with sodium phosphate (${\mathrm{Na}}_2{\mathrm{HPO}}_4$), the complete ionic equation displays all ions, while the net ionic equation focuses solely on the ions forming the precipitate.

Precipitation Reactions

Precipitation reactions occur when two soluble salts are mixed in solution and an insoluble salt, or precipitate, forms out of the solution. They are a type of double displacement reaction where the cations and anions exchange partners, resulting in the formation of a product that is no longer soluble in water.

In the case of our exercise, when ${\mathrm{MgSO}}_4$ is mixed with ${\mathrm{NH}}_4\mathrm{OH}$ and ${\mathrm{Na}}_2{\mathrm{HPO}}_4$, magnesium ions $({\mathrm{Mg}}^{2+})$ interact with phosphate ions $({\mathrm{PO}}_4^{3-})$ forming an insoluble compound $\mathrm{Mg}\left({\mathrm{NH}}_4\right){\mathrm{PO}}_4$. This compound, emerging as a solid from the solution, is the white crystalline precipitate mentioned in the exercise.

**Factors Affecting Precipitation**- **Concentration**: Higher concentrations can lead to more frequent collisions between ions, increasing the likelihood of precipitate formation.
- **Temperature**: Generally, increasing temperature can increase solubility, but specific reactions behave differently under thermal changes.
- **Nature of Ions**: Some ions form sturdy lattices, making it easier for them to fall out of solution as precipitates.

Stoichiometry

Stoichiometry is a fundamental concept in chemistry used to calculate the quantities of reactants and products involved in chemical reactions. It is based on the conservation of mass where the number of atoms of each element remains the same before and after the reaction. Performing stoichiometric calculations involves balancing the chemical equation and using mole ratios to predict the amounts of reactants needed or products formed.

For the provided reaction, knowing stoichiometry helps ensure that the balanced equation reflects the correct proportions of magnesium ions, ammonium ions, and phosphate ions in forming $\mathrm{Mg}\left({\mathrm{NH}}_4\right){\mathrm{PO}}_4$. The balanced reaction requires understanding that:

• Each ${\mathrm{Mg}}^{2+}$ ion requires one ${\mathrm{PO}}_4^{3-}$ ion and one ${\mathrm{NH}}_4^{+}$ ion to form the compound.
• The charges must balance to result in a neutral compound.
• The ratio of ions forming the precipitate determines the stoichiometry.

Stoichiometry helps ensure that reactions are treated quantitatively, predicting how much precipitate will form based on given reactants.