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 \(\mathrm{Y}\) 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) \(\mathrm{P}_{4} \mathrm{O}_{10}\) and \(\mathrm{HPO}_{3}\).

Step 1: Identify Possible Compound Y

Compound Y is formed from compound X after treatment with cold water and gives a white precipitate with BaCl₂. In an acidic medium, BaCl₂ typically precipitates out sulfate (SO₄²⁻) ions or phosphate (PO₄³⁻) ions. Compound Y can thus be a form of phosphoric acid that reacts with these.

Step 2: Check Reaction with AgNO₃

Compound Y gives a white precipitate with neutral AgNO₃. Silver nitrate in neutral solutions typically forms precipitates with various anions, including phosphate ions (like Ag₃PO₄ from phosphoric acids), which are white and amorphous.

Step 3: Identify Y from Thermal Decomposition Clue

Compound Y can also be obtained by heating H₃PO₄. Upon heating, H₃PO₄ can lose water to form HPO₃ (metaphosphoric acid). Thus, the compound Y is likely HPO₃.

Step 4: Deduce Compound X

Compound X, which forms Y upon adding cold water, should be an oxide of phosphorus that reacts with water to form HPO₃. When P₄O₁₀ (phosphorus pentoxide) reacts with water, it forms H₃PO₄, which dehydrates to HPO₃, matching compound Y’s characteristics.

Step 5: Select the Correct Option

Given P₄O₁₀ forms H₃PO₄ with water, which can further form HPO₃, the pair of compounds (X, Y) must be P₄O₁₀ and HPO₃. Reviewing the options, we see option (b) \(\mathrm{P}_{4} \mathrm{O}_{10}\) and \(\mathrm{HPO}_{3}\) matches the described transformations and precipitate formations.

Key Concepts

Phosphorus Oxides

Phosphorus oxides are compounds formed by the reaction of phosphorus with oxygen. They play critical roles in diverse chemical processes. The most well-known phosphorus oxides are phosphorus trioxide (\(\mathrm{P}_4\mathrm{O}_6\)) and phosphorus pentoxide (\(\mathrm{P}_4\mathrm{O}_{10}\)).

Phosphorus pentoxide, \(\mathrm{P}_4\mathrm{O}_{10}\), is a dry, white solid that is highly reactive.

• It is known for extreme hygroscopicity, meaning it readily absorbs water from the atmosphere.
• When it comes into contact with water, a reaction occurs to form phosphoric acids.

Its reactivity with water forms the basis for converting into various phosphoric acids based on the conditions and temperatures employed.

Phosphoric Acids

Phosphoric acids are essential in both laboratory and industrial chemistry. The simplest form is orthophosphoric acid, \(\mathrm{H}_3\mathrm{PO}_4\), an essential chemical in fertilizers and cleaning products.

Interestingly, heating \(\mathrm{H}_3\mathrm{PO}_4\) can result in the formation of metaphosphoric acid (\(\mathrm{HPO}_3\)):

• \(\mathrm{H}_3\mathrm{PO}_4\) upon heating loses water to yield \(\mathrm{HPO}_3\).
• Metaphosphoric acid is known to form certain white amorphous precipitates under the right conditions.

Due to their ability to form precipitates, phosphoric acids are used often in precipitation and detection reactions.

Precipitation Reactions

Precipitation reactions are key in identifying ions present in a solution. A precipitate is a solid that forms in a solution during a chemical reaction.

When compound \(\mathrm{Y}\) (possibly \(\mathrm{HPO}_3\)) is added to barium chloride (\(\mathrm{BaCl}_2\)) in acid, a white precipitate forms. This typically signifies the presence of phosphate ions:

• \(\mathrm{BaCl}_2\) in an acidic medium will precipitate out phosphate ions, forming \(\mathrm{Ba}_3(\mathrm{PO}_4)_2\).

Similarly, silver nitrate (\(\mathrm{AgNO}_3\)) in neutral conditions can also form silver phosphate (\(\mathrm{Ag}_3\mathrm{PO}_4\)), noted as a white amorphous precipitate. These predictable reactions are used extensively in qualitative analysis to pinpoint specific anions or cations in compound samples.

Chemical Reactions

Chemical reactions involve the transformation of substances through the breaking and forming of bonds, leading to new substances. In this exercise, phosphorous oxides and water react to create phosphoric acids.

The general process follows a pattern:

• When \(\mathrm{P}_4\mathrm{O}_{10}\) interacts with water, it forms \(\mathrm{H}_3\mathrm{PO}_4\).
• \(\mathrm{H}_3\mathrm{PO}_4\) can then dehydrate to form \(\mathrm{HPO}_3\).

This cycle of reactions emphasizes the important role of water in chemistry where it is not only a solvent but also a reagent that influences the synthesis of other substances such as phosphoric acids. This transformation is crucial for utilizing these compounds in various real-world applications.

Acid-Base Chemistry

Acid-base chemistry is a core area in Inorganic Chemistry that describes reactions between acids and bases. Acids such as phosphoric acid donate protons to bases, leading to neutralization reactions.

The behavior of phosphoric acids can vary based on their derivatives and the conditions in which they interact:

• \(\mathrm{H}_3\mathrm{PO}_4\) is a triprotic acid, meaning it can donate three protons.
• Each form of phosphoric acid, when reacting, showcases different acid-base properties, such as differing in how easily they release their protons.

Understanding these properties helps in working out the formation of salts or precipitates when specific reagents are used. This predictive power is foundational for synthetic and analytical uses in chemistry.