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

Question

When a white hygroscopic powder \(X\) is treated with cold water, a compound \(Y\) 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 \(\mathrm{Y}\) 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 \(\mathrm{HPO}_{3}\)

Step-by-Step Solution

Answer

(b) \(\mathrm{P_4O_{10}}\) and \(\mathrm{HPO_3}\)

Step 1: Identify the Reaction with Cold Water

The problem states that the hygroscopic powder \(X\) reacts with cold water to form compound \(Y\). Since \(Y\) forms a precipitate with \(\mathrm{BaCl_2}\) in an acidic medium, \(Y\) likely forms a sulfate, phosphate, or carbonate ion that can precipitate as \(\mathrm{BaSO_4}\), \(\mathrm{Ba_3(PO_4)_2}\), or \(\mathrm{BaCO_3}\). Given that \(X\) reacts with water to form \(\mathrm{H_3PO_4}\) and heating \(\mathrm{H_3PO_4}\) can produce \(\mathrm{HPO_3}\), option (b) seems correct.

Step 2: Determine the Precipitate with BaCl2

\(Y\), which gives a white precipitate with \(\mathrm{BaCl_2}\) in an acidic medium, means that \(Y\) likely forms \(\mathrm{Ba_3(PO_4)_2}\). This supports \(Y\) being \(\mathrm{HPO_3}\) as it provides \(\mathrm{PO_3}^{3-}\) ions to form a precipitate with \(\mathrm{BaCl_2}\).

Step 3: Determine the Precipitate with AgNO3

\(Y\) forms a white amorphous precipitate with neutral \(\mathrm{AgNO_3}\), which means \(Y\) must produce \(\mathrm{PO_3}^{3-}\) ions. This confirms \(Y\) as \(\mathrm{HPO_3}\) since phosphate ions give such a precipitate with silver nitrate.

Step 4: Confirmation through Heating H3PO4

The problem also says that heating \(\mathrm{H_3PO_4}\) can produce \(Y\). Heating \(\mathrm{H_3PO_4}\) yields \(\mathrm{HPO_3}\). This reinforces \(Y\) being \(\mathrm{HPO_3}\).

Step 5: Conclude the Compounds

Thus, \(X\), which reacts with water to form \(\mathrm{H_3PO_4}\), must be \(\mathrm{P_4O_{10}}\) because \(\mathrm{P_4O_{10}}\) is known to do so. Therefore, the compounds \(X\) and \(Y\) are \(\mathrm{P_4O_{10}}\) and \(\mathrm{HPO_3}\), respectively. Hence, the correct answer is (b).

Key Concepts

Hygroscopic Substances

Hygroscopic substances are materials that readily absorb moisture from their surroundings. They often have a strong affinity for water, drawing it from the air or other substances. This property is particularly important in various applications:

• Preservation: Hygroscopic substances can be used to maintain low moisture conditions, such as in packaging to prevent spoilage.
• Industrial Uses: These substances are essential in processes that require controlled humidity, like in the manufacturing of some pharmaceuticals and electronics.
• Dehumidifiers: Commonly used to keep environments dry, hygroscopic materials can help maintain comfort and prevent damage from excessive moisture.

In the context of our exercise, the white powder mentioned is hygroscopic, meaning it easily takes up water from the environment. It reacts with water, leading to chemical changes that result in another compound. This process illustrates how hygroscopic behavior is not merely about physical absorption but can also lead to new chemical products depending on the reactive nature of the substance.

Precipitation Reactions

Precipitation reactions occur when two solutions are mixed, resulting in the formation of a solid, known as a precipitate. This solid is insoluble in the solution, standing out clearly as it settles or forms a suspension in the liquid.

• Example: Mixing barium chloride ( \( \text{BaCl}_2 \) ) with sulfate ions ( \( \text{SO}_4^{2-} \) ) to form barium sulfate ( \( \text{BaSO}_4 \) ) is a classic precipitation reaction.
• Critical Conditions: The nature of the ions in solution, the temperature, and the pH of the solution impact whether a precipitate will form.
• Identification: Observation of a color change or cloudiness in the solution indicates a precipitation reaction has occurred.

In the presented exercise, the compound \( Y \) forms a precipitate both with \( \text{BaCl}_2 \) and \( \text{AgNO}_3 \). The formation of these precipitates is crucial for identifying \( Y \), as it helps indicate which ions \( Y \) provides in the reaction, giving valuable clues about its composition.

Phosphoric Acid Derivatives

Phosphoric acid derivatives are varied compounds that result from phosphoric acid (\( \text{H}_3\text{PO}_4 \)) through different chemical reactions. These derivatives often have diverse industrial and chemical significance:

• Formation: Such derivatives can be obtained by heating phosphoric acid, altering its composition and properties.
• Examples:
  • Metaphosphoric acid (\( \text{HPO}_3 \)): A result of heating \( \text{H}_3\text{PO}_4 \), used in various chemical applications for its dehydrated structure.
  • Phosphorous compounds: Compounds with different oxidation states of phosphorus have unique uses ranging from fertilizers to flame retardants.
• Properties: These derivatives can exhibit distinct reactivity and solubility, influencing their application in industrial chemistry and laboratory settings.

In our original exercise, heating \( \text{H}_3\text{PO}_4 \) produces \( \text{HPO}_3 \), a transformation showing how phosphoric acid derivatives change under thermal conditions and produce new functional compounds, assisting in confirming the identity of the final product.