The pair that contains two P—H bonds in each of the oxoacids is
(a) H4P2O5 and H4P2O6
(b) H3PO3 and

Q: Why doesn't $H_3PO_3$ have three -OH groups?

For stability in these oxoacids, phosphorus must form at least one $P=O$ double bond. In $H_3PO_3$, once you form the $P=O$ bond, you only have two oxygen atoms left, which can form a maximum of two $-OH$ groups. The third hydrogen must bond directly to the phosphorus.

Q: How do P-H bonds affect the properties of the acid?

Direct $P-H$ bonds are responsible for the reducing nature of the oxoacid. The more $P-H$ bonds an acid has, the stronger its reducing power. For example, $H_3PO_2$ (two $P-H$ bonds) is a stronger reducing agent than $H_3PO_3$ (one $P-H$ bond).

Q: Are P-H bonds ionizable in water?

No, $P-H$ bonds are not ionizable because the electronegativity difference between phosphorus and hydrogen is very small. Only the hydrogen atoms attached to highly electronegative oxygen atoms (in $-OH$ groups) can ionize to release $H^+$ ions.

Q: What is the basicity of $H_3PO_2$?

Even though $H_3PO_2$ has three hydrogen atoms, it is a monobasic acid. This is because only one hydrogen is part of an $-OH$ group and can be ionized. The other two are directly attached to phosphorus.

Q: How can I easily remember the structures of dimers like $H_4P_2O_5$?

Think of them as anhydrides. $H_4P_2O_5$ (pyrophosphorous acid) can be visualized as taking two molecules of $H_3PO_3$ and removing one molecule of water ($H_2O$) from them, resulting in a $P-O-P$ bridge.

Question

The pair that contains two P—H bonds in each of the oxoacids is

Options:

(A) $H_4P_2O_5$ and $H_4P_2O_6$

(B) $H_3PO_3$ and $H_3PO_2$

(C) $H_4P_2O_5$ and $H_3PO_3$

(D) $H_3PO_2$ and $H_4P_2O_5$

Sigma Percentile · Accepted Answer

### The Architecture of Phosphorus Oxoacids

When dealing with the oxoacids of phosphorus, memorizing formulas is a recipe for disaster. The true power lies in understanding their structural architecture. Phosphorus, being a Group 15 element, typically exhibits a $+5$, $+3$, or $+1$ oxidation state in these acids. However, regardless of the oxidation state, there is a fundamental structural rule that almost all phosphorus oxoacids obey: **The central phosphorus atom is tetrahedrally surrounded by other atoms, and it must form at least one $P=O$ double bond and at least one $P-OH$ single bond.**

Once you establish this $P=O$ and $P-OH$ skeleton, the remaining valencies of phosphorus are satisfied by either additional $-OH$ groups or direct $P-H$ bonds, depending on how many oxygen and hydrogen atoms are left in the molecular formula.

### Analyzing the Candidates

Let's break down the structures of the molecules given in the options to count their direct $P-H$ bonds.

**1. Hypophosphorous Acid ($H_3PO_2$)**
We start with the skeleton: one $P=O$ and one $P-OH$. This consumes one P, two O's, and one H. We are left with two hydrogen atoms. Since there are no more oxygen atoms available to form $-OH$ groups, these two hydrogens must bond directly to the central phosphorus atom. 
Thus, $H_3PO_2$ has exactly **two $P-H$ bonds**. Because it only has one ionizable $-OH$ group, it is a monobasic acid.

**2. Orthophosphorous Acid ($H_3PO_3$)**
Again, start with the skeleton: one $P=O$ and one $P-OH$. We have used one P, two O's, and one H. We are left with one oxygen and two hydrogens. The remaining oxygen pairs with one hydrogen to form a second $-OH$ group. The final hydrogen has no choice but to bond directly to the phosphorus. 
Therefore, $H_3PO_3$ has exactly **one $P-H$ bond**. It is a dibasic acid.

**3. Pyrophosphorous Acid ($H_4P_2O_5$)**
This is a dimer. The prefix "pyro-" often indicates an anhydride formed by the loss of a water molecule from two molecules of the parent acid (in this case, $H_3PO_3$). The structure features a central $P-O-P$ bridge. Each phosphorus atom in this bridge maintains the standard skeleton: a $P=O$ bond and a $P-OH$ bond. This leaves one hydrogen atom for each phosphorus to bond with directly. 
Consequently, the entire $H_4P_2O_5$ molecule contains **two $P-H$ bonds** (one on each phosphorus atom).

**4. Hypophosphoric Acid ($H_4P_2O_6$)**
This molecule is unique. Instead of an oxygen bridge, it features a direct $P-P$ bond. Each phosphorus atom is bonded to one $=O$ and two $-OH$ groups. All four hydrogen atoms in the formula are part of hydroxyl groups. 
As a result, $H_4P_2O_6$ has **zero $P-H$ bonds**.

### The Final Verdict

Our objective was to find the pair where *both* oxoacids contain exactly two $P-H$ bonds. Based on our structural analysis:
- $H_3PO_2$ has 2 $P-H$ bonds.
- $H_4P_2O_5$ has 2 $P-H$ bonds.

Therefore, the correct pair is **$H_3PO_2$ and $H_4P_2O_5$**, making option (d) the correct answer. The presence of these direct $P-H$ bonds is not just a structural trivia; it is the very reason why these specific acids act as strong reducing agents in chemical reactions!

Animated Solution for Chemistry - s and p-Block Elements: The pair that contains two P—H bonds in each of the oxoacids is

Select Answer:

Visualized Solution

\text{Analyzing Oxoacids of Phosphorus}

H_3PO_2 \text{ (Hypophosphorous Acid)}

H_3PO_3 \text{ (Orthophosphorous Acid)}

H_4P_2O_5 \text{ (Pyrophosphorous Acid)}

H_4P_2O_6 \text{ (Hypophosphoric Acid)}

\text{Final Conclusion}

The Sigma Insight: Group 15 Elements

The Architecture of Phosphorus Oxoacids

Analyzing the Candidates

The Final Verdict

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