Question

The number of non-ionisable hydrogen atoms present in the final product obtained from the hydrolysis of PCl\(_5\) is :

• A. 1
• B. 2
• C. 3
• D. 0

Hint:

For phosphorus oxyacids, remember the rule for basicity (number of ionisable H atoms):
H\(_3\)PO\(_4\) (Phosphoric acid): All 3 H are on O atoms. Basicity = 3.
H\(_3\)PO\(_3\) (Phosphorous acid): 2 H are on O atoms, 1 H is on P.
Basicity = 2.
H\(_3\)PO\(_2\) (Hypophosphorous acid): 1 H is on an O atom, 2 H are on P. Basicity = 1.
Only hydrogens attached to oxygen are acidic.

Answer 1

The Correct Option is D

Step 1: Understanding the Question
The question asks for the number of non-ionisable hydrogen atoms in the product formed when phosphorus pentachloride (PCl\(_5\)) undergoes complete hydrolysis.
Step 2: Writing the Hydrolysis Reaction
Phosphorus pentachloride (PCl\(_5\)) reacts with water in a two-step process, but the final product of complete hydrolysis is phosphoric acid (H\(_3\)PO\(_4\)) and hydrochloric acid (HCl).
The overall reaction is:
\[ \text{PCl}_5 + 4\text{H}_2\text{O} \rightarrow \text{H}_3\text{PO}_4 + 5\text{HCl} \] The final product we need to analyze is phosphoric acid, H\(_3\)PO\(_4\).
Step 3: Determining Ionisable and Non-ionisable Hydrogens
In oxyacids, hydrogen atoms are ionisable (acidic) if they are bonded to a highly electronegative atom, typically oxygen. Hydrogen atoms bonded directly to the central, less electronegative atom (like phosphorus) are generally non-ionisable.
\[ \begin{array}{c} \text{OH}
|
\text{HO} - \text{P} - \text{OH}
||
\text{O} \end{array} \] In the structure of H\(_3\)PO\(_4\), the central phosphorus atom is double-bonded to one oxygen atom and single-bonded to three hydroxyl (-OH) groups. All three hydrogen atoms are attached to oxygen atoms.
Since all three hydrogens are part of -OH groups, they can be released as H\(^+\) ions in an aqueous solution. Therefore, all three hydrogen atoms are ionisable.
Step 4: Final Answer
The number of non-ionisable hydrogen atoms in H\(_3\)PO\(_4\) is zero.