The compound(s) with P-H bond(s) is(are)
(A) H3PO4
(B) H3PO3
(C) H4P2O7
(D) H3PO2

Q: Why aren't all hydrogen atoms in $H_3PO_3$ acidic?

Only the hydrogen atoms attached to the highly electronegative oxygen atoms (in $P-OH$ groups) can be released as protons ($H^+$). The hydrogen attached directly to phosphorus is not acidic because the $P-H$ bond is almost non-polar and does not easily ionize in water.

Q: How can I quickly determine the basicity of a phosphorus oxoacid from its formula?

A good rule of thumb for simple mononuclear oxoacids ($H_3PO_x$) is that the basicity is usually $x - 1$. For example, $H_3PO_4$ has a basicity of 3, $H_3PO_3$ has a basicity of 2, and $H_3PO_2$ has a basicity of 1.

Q: What makes a phosphorus oxoacid a good reducing agent?

The presence of direct $P-H$ bonds is responsible for the reducing nature. These bonds are relatively weak and can easily undergo oxidation, allowing the compound to act as a reducing agent. $H_3PO_2$ is a stronger reducing agent than $H_3PO_3$ because it has two $P-H$ bonds compared to one.

Q: What is the oxidation state of phosphorus in $H_3PO_2$?

Let the oxidation state of phosphorus be $x$. The sum of oxidation states must be zero: $3(+1) + x + 2(-2) = 0$. Solving this gives $3 + x - 4 = 0$, which means $x = +1$.

Q: How is the structure of $H_4P_2O_7$ derived?

It is a 'pyro' acid, meaning it is formed by heating two molecules of the 'ortho' acid ($H_3PO_4$) and removing one molecule of water ($H_2O$). This dehydration process creates a $P-O-P$ bridge connecting the two phosphorus atoms.

Question

The compound(s) with P–H bond(s) is(are)

Options:

(A) $H_3PO_4$

(B) $H_3PO_3$

(C) $H_4P_2O_7$

(D) $H_3PO_2$

Sigma Percentile · Accepted Answer

The chemistry of p-block elements is filled with fascinating structural nuances, and the oxoacids of phosphorus are a prime example of this. When faced with a question asking to identify the presence of direct $P-H$ bonds, the most reliable strategy is to draw the Lewis structures of the given molecules. Let's embark on a structural journey to decode this problem.

### The Golden Rules of Phosphorus Oxoacids

Before we dive into the specific options, it is crucial to understand the fundamental building blocks of these acids. In all stable oxoacids of phosphorus, the central phosphorus atom is $sp^3$ hybridized, adopting a tetrahedral geometry.

There are two non-negotiable structural rules you must always remember:
1. Every phosphorus atom must form at least one double bond with an oxygen atom ($P=O$).
2. Every phosphorus atom must form at least one single bond with a hydroxyl group ($P-OH$).

Once you satisfy these two conditions, you distribute the remaining oxygen and hydrogen atoms. Any hydrogen atom that cannot be paired with an oxygen to form a hydroxyl group must bond directly to the central phosphorus atom, creating a $P-H$ bond.

### Analyzing the Candidates

Let's apply our golden rules to the options provided in the question.

**Option (A): Orthophosphoric acid ($H_3PO_4$)**
We start with the central $P$ atom. We assign one $P=O$ bond. This leaves us with three oxygen atoms and three hydrogen atoms. Perfectly, they pair up to form three $P-OH$ groups. The structure is completely satisfied, and as we can see, all hydrogen atoms are bonded to highly electronegative oxygen atoms. There are **zero** $P-H$ bonds.

**Option (B): Phosphorous acid ($H_3PO_3$)**
Again, we begin with the mandatory $P=O$ bond. We are left with two oxygen atoms and three hydrogen atoms. Two hydrogens pair with the two oxygens to form two $P-OH$ groups. We have one hydrogen atom left over! This lone hydrogen has no choice but to bond directly to the phosphorus atom. Thus, $H_3PO_3$ contains **one** $P-H$ bond. This direct $P-H$ bond is what gives phosphorous acid its reducing properties.

**Option (C): Pyrophosphoric acid ($H_4P_2O_7$)**
The prefix 'pyro' indicates that this acid is formed by heating. Specifically, it is a dimer created by condensing two molecules of orthophosphoric acid ($H_3PO_4$) with the loss of one water molecule ($H_2O$). This condensation creates a bridging $P-O-P$ linkage. Each phosphorus atom in the dimer maintains one $P=O$ bond and two $P-OH$ bonds. Consequently, there are **zero** $P-H$ bonds in this structure.

**Option (D): Hypophosphorous acid ($H_3PO_2$)**
Following our rules, we establish the $P=O$ bond. We are now left with only one oxygen atom but three hydrogen atoms. One hydrogen pairs with the single oxygen to form one $P-OH$ group. The remaining two hydrogen atoms must bond directly to the central phosphorus. Therefore, $H_3PO_2$ contains **two** $P-H$ bonds, making it an exceptionally strong reducing agent.

### The Final Verdict

By systematically drawing and analyzing the structures, the answer becomes crystal clear. Phosphorous acid ($H_3PO_3$) and hypophosphorous acid ($H_3PO_2$) are the only compounds among the choices that possess direct phosphorus-hydrogen bonds.

Therefore, the correct options are **(B)** and **(D)**. This problem beautifully illustrates why memorizing formulas is never enough in chemistry; true mastery lies in understanding the underlying molecular architecture.

Animated Solution for Chemistry - s and p-Block Elements: The compound(s) with P–H bond(s) is(are)

Select Answer:

Visualized Solution

Visual Anchor: The Oxoacids of Phosphorus

Analyzing $H_{3} P O_{4}$

Analyzing $H_{3} P O_{3}$

Analyzing $H_{4} P_{2} O_{7}$

Analyzing $H_{3} P O_{2}$

Final Conclusion

The Sigma Insight: Group 15 Elements

The Golden Rules of Phosphorus Oxoacids

Analyzing the Candidates

The Final Verdict

Similar Questions

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

In a molecule of pyrophosphoric acid, the number of $P—OH$ , $P = O$ and $P—O—P$ bonds/moiety(ies) respectively are

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Which of the following statements is wrong?