Problem 145 - A Complete Resource Book in Chemistry for JEE Main

Question

When a white hygroscopic powder \(\mathrm{X}\) is treated with cold water, a compound \(\mathrm{Y}\) is formed which forms a white precipitate with \(\mathrm{BaCl}_{2}\) in acidic medium. \(\mathrm{Y}\) also gives a white amorphous precipitate with neutral \(\mathrm{AgNO}_{3} .\) Compound Y can also be obtained by heating \(\mathrm{H}_{3} \mathrm{PO}_{4}\) here \(\mathrm{X}\) and \(\mathrm{Y}\) are respectively (a) \(\mathrm{P}_{4} \mathrm{O}_{10}\) and \(\mathrm{H}_{3} \mathrm{PO}_{2}\) (b) \(\mathrm{P}_{4} \mathrm{O}_{10}\) and \(\mathrm{HPO}_{3}\) (c) \(\mathrm{P}_{4} \mathrm{O}_{6}\) and \(\mathrm{H}_{3} \mathrm{PO}_{2}\) (d) \(\mathrm{N}_{2} \mathrm{O}_{5}\) and \(\mathrm{HPO}_{2}\)

Step-by-Step Solution

Answer

The answer is (b) \(\text{P}_4\text{O}_{10}\) and \(\text{HPO}_3\).

Step 1: Identify possible reactions with given hints

First, analyze the behavior of compound Y with other reagents given in the problem. The problem states that Y gives a white precipitate with \(\text{BaCl}_2\) in an acidic medium. This indicates that compound Y contains a phosphate (\(\text{PO}_4^{3-}\)) or a pyrophosphate (\(\text{P}_2\text{O}_7^{4-}\)) group because these ions form a white barium salt in the presence of barium ions. Further, Y reacts with \(\text{AgNO}_3\) to form a white amorphous precipitate, which hints at either a phosphate or a polyphosphate group in compound Y.

Step 2: Determine compound Y by heat formation data

Refer to the additional information that compound Y can be obtained by heating \(\text{H}_3\text{PO}_4\). Heating orthophosphoric acid (\(\text{H}_3\text{PO}_4\)) leads to the formation of metaphosphoric acid (\(\text{HPO}_3\)) by dehydration. This means that compound Y is likely \(\text{HPO}_3\), as it is formed by heating \(\text{H}_3\text{PO}_4\).

Step 3: Identify compound X and confirm the solution

Given that compound X forms compound Y with cold water, X should be a dehydrating agent that, upon hydration, gives \(\text{HPO}_3\). \(\text{P}_4\text{O}_{10}\) is the anhydride of phosphoric acids and when reacting with water, initially forms \(\text{HPO}_3\). Therefore, compound X is \(\text{P}_4\text{O}_{10}\).

Step 4: Answer elimination

Now, compare the derived compounds with the options given: (a) \(\text{P}_4\text{O}_{10}\) and \(\text{H}_3\text{PO}_2\), (b) \(\text{P}_4\text{O}_{10}\) and \(\text{HPO}_3\), (c) \(\text{P}_4\text{O}_6\) and \(\text{H}_3\text{PO}_2\), and (d) \(\text{N}_2\text{O}_5\) and \(\text{HPO}_2\). The correct answer, matching our analysis, is (b) \(\text{P}_4\text{O}_{10}\) and \(\text{HPO}_3\).

Key Concepts

Phosphorus Oxides

Phosphorus oxides are important compounds in chemistry and have diverse applications. The most common ones include

• Phosphorus pentoxide ( P_{4}O_{10} ), which is also known as tetraphosphorus decoxide.
• Phosphorus trioxide ( P_{4}O_{6} ), which is less commonly encountered.

Phosphorus pentoxide acts as a powerful desiccant, which means it is very efficient in absorbing water. When P_{4}O_{10} interacts with water, it undergoes a series of reactions to form phosphoric acids. Initially, it forms metaphosphoric acid ( HPO_{3} ), explaining its use in the given exercise to generate compound Y. This characteristic makes it very significant in various industrial applications, especially where water absorption is crucial.

Phosphate Chemistry

Phosphate chemistry focuses on the compounds containing the phosphate anion ( PO_{4}^{3-} ). Phosphates are essential in multiple chemical processes for industrial, agricultural, and biological purposes.

• Phosphoric acid derivatives, like orthophosphoric acid ( H_{3}PO_{4} ), are key reagents in many reactions.
• Metaphosphoric acid ( HPO_{3} ) results from the dehydration of H_{3}PO_{4} , especially when heated.
• Polyphosphates can include pyrophosphates ( P_{2}O_{7}^{4-} ), another important class within phosphate chemistry.

These compounds serve distinct roles in transportation of energy within cells (like ATP in biochemistry), as well as being used in detergents and fertilizers. In the context of the exercise, the ability of compound Y ( HPO_{3} ) to precipitate as a white solid with specific conditions (e.g., BaCl_{2} in acidic medium) suggests it involves such ionic phosphate species.

Precipitation Reactions

Precipitation reactions involve the formation of a solid, called a precipitate, when two aqueous solutions are mixed. The precipitate forms when the product of the ionic concentrations in solution exceeds the solubility of the compound in water.

• This type of reaction is useful for identifying the composition of unknown solutions based on precipitate formation.
• In our exercise, compound Y reacts with BaCl_{2} and AgNO_{3} , both of which form respective barium and silver phosphate precipitates.
• These reactions are usually confirmed by a characteristic appearance of a solid in the solution, often accompanied by a change in the solution's clarity.

Scientific techniques like these are widely used in laboratory settings to detect the presence of specific ions. Through the careful observation of precipitates, chemists can draw conclusions about the compounds present in a reaction, providing foundational techniques used in qualitative analysis.