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 white crystalline precipitate formed is \( \mathrm{Mg}\left(\mathrm{NH}_{4}\right) \mathrm{PO}_{4} \).

Step 1: Understand the Reaction

The problem states that Magnesium sulfate \( \mathrm{MgSO}_{4} \) reacts with Ammonium hydroxide \( \mathrm{NH}_{4} \mathrm{OH} \) and Sodium hydrogen phosphate \( \mathrm{Na}_{2} \mathrm{HPO}_{4} \) to form a white crystalline precipitate. We need to determine the composition of this precipitate.

Step 2: Identify Possible Precipitates

The options provided list possible compounds that could precipitate from the reaction. We need to analyze each option: 1. \( \mathrm{Mg}\left(\mathrm{NH}_{4}\right) \mathrm{PO}_{4} \) 2. \( \mathrm{Mg}_{3}\left(\mathrm{PO}_{4}\right)_{2} \) 3. \( \mathrm{MgCl}_{2} \cdot \mathrm{MgSO}_{4} \) 4. \( \mathrm{MgSO}_{4} \) again (which would not form since that's a starting reactant).

Step 3: Predict Reaction Products

\( \mathrm{MgSO}_{4} \) and \( \mathrm{Na}_{2} \mathrm{HPO}_{4} \) can react with \( \mathrm{NH}_{4} \mathrm{OH} \) to produce mixtures containing various possible precipitates. \( \mathrm{Mg}\left(\mathrm{NH}_{4}\right) \mathrm{PO}_{4} \) is one likely candidate, as it uses all components present (Mg, NH4, and PO4).

Step 4: Verify Stoichiometry

Ensure that the proposed precipitate \( \mathrm{Mg}\left(\mathrm{NH}_{4}\right) \mathrm{PO}_{4} \) balances stoichiometrically with the given starting reagents. Both \( \mathrm{NH}_{4} \) and \( \mathrm{PO}_{4} \) appear once each per \( \mathrm{Mg} \) atom, which matches well with the reagent availability.

Step 5: Compare with Given Options

Among the options, \( \mathrm{Mg}\left(\mathrm{NH}_{4}\right) \mathrm{PO}_{4} \) correctly reflects the products observed in similar experiments, forming a white crystalline precipitate. The stoichiometry and chemical composition align with practical chemistry.

Key Concepts

Understanding Magnesium Sulfate

Magnesium sulfate is a salt often encountered in chemical experiments. Its chemical formula is \( \mathrm{MgSO}_{4} \).
This compound is well-known for its solubility in water, but, in some reactions, it can lead to the formation of precipitates.
Magnesium sulfate consists of a magnesium ion \( (\mathrm{Mg}^{2+}) \) and a sulfate ion \( (\mathrm{SO}_{4}^{2-}) \).
These ions play crucial roles in determining the kinds of chemical reactions \( \mathrm{MgSO}_{4} \) can participate in, especially precipitation reactions.
Precipitation occurs when ions in solution form a solid. This happens when the product of their concentrations exceeds the solubility product.

The Role of Ammonium Hydroxide

Ammonium hydroxide \( \mathrm{NH}_{4} \mathrm{OH} \) is an important reagent.
It introduces ammonium ions \( (\mathrm{NH}_{4}^{+}) \) and hydroxide ions \( (\mathrm{OH}^{-}) \) into a solution.
Both ions can participate in a variety of reactions depending on the other chemicals present.

• Ammonium ions \( (\mathrm{NH}_{4}^{+}) \) can combine with other anions to form compounds.
• Hydroxide ions can prompt reactions resulting in precipitation, especially with metal cations.

In the context of this exercise, ammonium hydroxide reacts with other components to help form the white crystalline precipitate.
Understanding how \( \mathrm{NH}_{4} \mathrm{OH} \) influences this reaction is key to predicting its products.

Role of Sodium Hydrogen Phosphate

Sodium hydrogen phosphate \( \mathrm{Na}_{2} \mathrm{HPO}_{4} \) is a source of phosphate ions \( (\mathrm{PO}_{4}^{3-}) \).
The introduction of phosphate ions can lead to the formation of new compounds when they interact with available cations in the solution.

• Phosphate ions readily combine with metal ions, such as magnesium, to form precipitates.
• This transformation is key to the reaction discussed in the exercise, leading to potential compounds like \( \mathrm{Mg}\left(\mathrm{NH}_{4}\right) \mathrm{PO}_{4} \).

Sodium hydrogen phosphate provides the phosphate needed in the precipitation of magnesium ammonium phosphate, the probable precipitate in this scenario.

Chemical Stoichiometry in Reaction Predictions

Chemical stoichiometry involves the quantitative relationship between reactants and products in a chemical reaction.
It helps predict the amounts and identities of products formed.
Stoichiometry uses balanced chemical equations to ensure conservation of mass and charge.
In the given reaction:

• Magnesium sulfate, ammonium hydroxide, and sodium hydrogen phosphate mix to potentially form various compounds.
• The proposed precipitate, magnesium ammonium phosphate \( \mathrm{Mg}\left(\mathrm{NH}_{4}\right) \mathrm{PO}_{4} \), aligns well with initial reactants.
• Stoichiometrically, each magnesium is matched with one ammonium and one phosphate, underscoring the likelihood of this product.

Thus, stoichiometry provides a reliable method for predicting which compounds will form based on reagent interactions and availability.