Problem 104 - A Complete Resource Book in Chemistry for JEE Main

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) \(\mathrm{MgSO}_{4}\)

Step-by-Step Solution

Answer

The formula of the precipitate is \(\mathrm{Mg}\left(\mathrm{NH}_{4}\right) \mathrm{PO}_{4}\) (option a).

Step 1: Identify the Reaction Components

We have magnesium sulfate (\(\mathrm{MgSO}_{4}\)), ammonium hydroxide (\(\mathrm{NH}_{4} \mathrm{OH}\)), and sodium hydrogen phosphate (\(\mathrm{Na}_{2} \mathrm{HPO}_{4}\)) as reactants. These will react to form a precipitate.

Step 2: Determine the Potential Product

Knowing common chemical reactions, \(\mathrm{MgSO}_{4}\) in the presence of \(\mathrm{NH}_{4} \mathrm{OH}\) and a phosphate ion source like \(\mathrm{Na}_{2} \mathrm{HPO}_{4}\) typically forms a magnesium phosphate compound.

Step 3: Analyze Precipitate Formation

When magnesium ions (\(\mathrm{Mg}^{2+}\)) react with ammonium phosphate ions, one of the common precipitates is magnesium ammonium phosphate \(\mathrm{Mg}\left(\mathrm{NH}_{4}\right)\mathrm{PO}_{4}\).

Step 4: Eliminate Other Options

Check each option: \(\mathrm{Mg}\left(\mathrm{NH}_{4}\right) \mathrm{PO}_{4}\) matches a known precipitate. \(\mathrm{Mg}_{3}\left(\mathrm{PO}_{4}\right)_{2}\) does not typically form in this condition. \(\mathrm{MgCl}_{2} \cdot \mathrm{MgSO}_{4}\) and \(\mathrm{MgSO}_{4}\) aren't formed by these reactions.

Step 5: Conclude the Correct Answer

The correct formula for the white crystalline precipitate formed is \(\mathrm{Mg}\left(\mathrm{NH}_{4}\right) \mathrm{PO}_{4}\), corresponding to option (a).

Key Concepts

Precipitation Reactions

A precipitation reaction occurs when two solutions are mixed, and an insoluble solid known as a precipitate forms. This process is a cornerstone of inorganic chemistry. When specific ions combine, they form compounds that are insoluble in the reaction mixture, falling out of the solution as a solid. This is particularly evident in reactions involving metal ions and anions like sulfates or phosphates. To predict the formation of a precipitate, solubility rules are often employed. These guidelines help determine which ionic compounds will not dissolve in water. In the context of the problem, the reaction between magnesium sulfate, ammonium hydroxide, and sodium hydrogen phosphate leads to the formation of an insoluble compound, magnesium ammonium phosphate, illustrating a classic precipitation reaction.

Magnesium Compounds

Magnesium, a group 2 alkaline earth metal, forms a variety of compounds important in both inorganic and organic chemistry. When reacting with other elements, magnesium typically forms compounds in the 2+ oxidation state. Some common magnesium compounds include magnesium chloride (\(\mathrm{MgCl}_{2}\)), magnesium sulfate (\(\mathrm{MgSO}_{4}\)), and magnesium oxide (\(\mathrm{MgO}\)).
Magnesium sulfate is particularly notable for its high water solubility, making it suitable for use in various aqueous reactions. Conversely, when magnesium forms compounds like phosphates, they tend to be less soluble. This decreased solubility is often a key factor in the formation of a precipitate. In our specific case, magnesium combines with ammonium phosphate ions to create the solid compound magnesium ammonium phosphate, \(\mathrm{Mg}\left(\mathrm{NH}_{4}\right)\mathrm{PO}_{4}\), which precipitates out of the solution.

Phosphate Ions

Phosphate ions (\(\mathrm{PO}_{4}^{3-}\)) are a common group of polyatomic ions in chemistry that feature prominently in a variety of biological and chemical processes. They can combine with metals to form a wide range of insoluble salts. The formation of these insoluble salts is what often drives precipitation reactions.
When a phosphate ion reacts with magnesium ions in solution, the result is the formation of magnesium phosphate compounds. In aqueous solutions, these phosphates are often less soluble, leading to the formation of a precipitate. The specific reaction in our exercise demonstrates how phosphate ions interact with magnesium ions to form magnesium ammonium phosphate. This compound crystallizes out of the solution as a white solid, a characteristic precipitation product in many similar chemical processes.