Problem 28 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

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? \([2006-5 \mathrm{M},-1]\) (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 \(\mathrm{Mg}\left(\mathrm{NH}_{4}\right)\mathrm{PO}_{4}\).

Step 1: Identify Reactants and Products

The reaction involves \ \( \mathrm{MgSO}_{4} \) \ (magnesium sulfate), \( \mathrm{NH}_{4} \mathrm{OH} \) \ (ammonium hydroxide), and \( \mathrm{Na}_{2} \mathrm{HPO}_{4} \) \ (disodium hydrogen phosphate). These react to form a white crystalline precipitate.

Step 2: Write Possible Reaction Equation

The reaction that forms a white crystalline precipitate is best explained by the formation of \( \mathrm{MgNH}_{4}\mathrm{PO}_{4} \). This compound is a common precipitate in such reactions involving magnesium ions and phosphate ions.

Step 3: Check for Precipitate Match

The possible products based on the given choices are \( \mathrm{Mg}\left(\mathrm{NH}_{4}\right) \mathrm{PO}_{4} \), \( \mathrm{Mg}_{3}\left(\mathrm{PO}_{4}\right)_{2} \), \( \mathrm{MgCl}_{2} \cdot \mathrm{MgSO}_{4} \), and \( \mathrm{MgSO}_{4} \). Of these, only \( \mathrm{MgNH}_{4}\mathrm{PO}_{4} \) is known to be a white crystalline precipitate in such reactions.

Step 4: Identify Correct Answer

Based on the knowledge of common magnesium phosphates, the white crystalline precipitate formed in this reaction is \( \mathrm{Mg}\left(\mathrm{NH}_{4}\right) \mathrm{PO}_{4} \). Thus, the correct answer is option (a) \( \mathrm{Mg}\left(\mathrm{NH}_{4}\right) \mathrm{PO}_{4} \).

Key Concepts

Magnesium Sulfate Reactions

Magnesium sulfate, represented by the chemical formula \( \mathrm{MgSO}_4 \), is a versatile inorganic salt with various reactions in chemistry. It typically dissolves in water to form magnesium and sulfate ions. When it reacts with other chemicals, the type of reaction and the products formed can vary. In our exercise, it's important to understand how \( \mathrm{MgSO}_4 \) interacts with compounds like ammonium hydroxide \( \mathrm{NH}_4\mathrm{OH} \) and disodium hydrogen phosphate \( \mathrm{Na}_2\mathrm{HPO}_4 \). These reactions can often result in the formation of different types of salts or precipitates, depending on the ions involved.

When \( \mathrm{MgSO}_4 \) reacts with \( \mathrm{NH}_4\mathrm{OH} \) and \( \mathrm{Na}_2\mathrm{HPO}_4 \), a white precipitate forms. This is due to the formation of magnesium ammonium phosphate, \( \mathrm{MgNH}_4\mathrm{PO}_4 \). Some of the underlying concepts here include:

• The role of magnesium ions in forming stable compounds with phosphate ions.
• The conditions under which these reactions occur, such as the presence of aqueous solutions and the correct reactant ratios.

Understanding these reactions is crucial as they are foundational to many inorganic chemistry problems, especially those involving ionic exchange and precipitation.

Ammonium Phosphate Precipitate

When it comes to inorganic chemistry, precipitates are formed when two soluble salts react in solution to form an insoluble product. In our problem, the focus is on the formation of ammonium phosphate precipitate, specifically magnesium ammonium phosphate \( \mathrm{Mg(\text{NH}_4)PO}_4 \). This compound is known for forming a white crystalline solid when the specific reactants are mixed.

The reaction can be described as the combination of magnesium ions \( \mathrm{Mg}^{2+} \) from magnesium sulfate and phosphate ions \( \mathrm{PO}_4^{3-} \) from disodium hydrogen phosphate. Additionally, ammonium ions \( \mathrm{NH}_4^{+} \) are introduced into the mix, leading to the formation of the solid precipitate.

• This process demonstrates the principle of ionic precipitation, a common occurrence in aqueous chemistry.
• Understanding why certain ions preferentially combine to form a solid is key to mastering reaction predictions and analysis skill in chemistry.

By recognizing why \( \mathrm{MgNH}_4\mathrm{PO}_4 \) forms instead of other potential products, students gain deeper insights into ionic interactions and compound solubility.

Inorganic Chemistry Reactions

Inorganic chemistry covers a vast array of reactions involving non-organic compounds. These reactions often include the exchange of ions in solutions, leading to the formation of various precipitates, gases, or other compounds. The exercise concerning magnesium sulfate and its reaction with other compounds is an excellent example.

In these types of reactions, the key factors to consider are

• Ion Exchange: Ions from different compounds swap partners to form new compounds. This is what happens when magnesium exchanges with ammonium and phosphate ions.
• Solubility Rules: Knowing which compounds are soluble and which form precipitates is essential. Inorganic chemistry relies heavily on these rules to predict reaction outcomes.
• Balancing Reactions: Ensuring the chemical equation is balanced in terms of mass and charge is fundamental for correct analysis.

These concepts apply not only to magnesium sulfate reactions but to a broader spectrum of inorganic reactions. Mastering them allows students to anticipate and understand complex reaction pathways, crucial for success in chemistry exams like JEE.