In a molecule of pyrophosphoric acid, the number of P-OH, P=O and P-O-P bonds/moiety(ies) respective

Q: Why does phosphorus form 5 bonds here? Doesn't it violate the octet rule?

Phosphorus is a Period 3 element, which means it has empty 3d orbitals available in its valence shell. It can promote an electron from the 3s orbital to the 3d orbital, allowing it to form 5 covalent bonds (an expanded octet). This is very common for elements in Period 3 and beyond.

Q: How do I easily remember the formula of pyrophosphoric acid?

Just remember the 'pyro' rule: take two molecules of the 'ortho' acid and subtract one water molecule. So, $2 \times H_3PO_4 = H_6P_2O_8$. Subtract $H_2O$, and you get $H_4P_2O_7$.

Q: Are all the P-OH bonds ionizable in water?

Yes! The hydrogen atoms attached to the highly electronegative oxygen atoms are acidic. Since there are four $P-OH$ groups, pyrophosphoric acid is a tetrabasic acid, meaning it can donate up to four protons ($H^+$) in an aqueous solution.

Q: Why is there a P-O-P bond instead of a direct P-P bond?

The reaction forming pyrophosphoric acid is a condensation reaction between two $-OH$ groups from different orthophosphoric acid molecules. When they react, $H_2O$ is eliminated, leaving behind a single oxygen atom that bridges the two phosphorus atoms.

Q: What is the oxidation state of phosphorus in this molecule?

Let's calculate it: $4(+1) + 2(x) + 7(-2) = 0$. Solving for $x$, we get $2x - 10 = 0$, so $x = +5$. The oxidation state is +5, which is the same as in orthophosphoric acid.

Question

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

Options:

(A) 2, 4 and 1

(B) 3, 3, and 3

(C) 4, 2, and 0

(D) 4, 2, and 1

Sigma Percentile · Accepted Answer

### The "Pyro" Connection

When we encounter the prefix **"pyro-"** in inorganic chemistry, it usually hints at a process involving heat. Pyrophosphoric acid is no exception. Imagine taking two molecules of orthophosphoric acid ($H_3PO_4$) and heating them. The thermal energy forces them to undergo a condensation reaction, where they lose a single water molecule ($H_2O$) and fuse together.

Mathematically, this looks like:
$$2H_3PO_4 \xrightarrow{\Delta} H_4P_2O_7 + H_2O$$

This resulting molecule, $H_4P_2O_7$, is our pyrophosphoric acid. Understanding this origin story makes visualizing its structure incredibly intuitive.

### Decoding the Structure

Let's build the molecule from the ground up. Since it was formed by joining two phosphoric acid units via the removal of water, the two central phosphorus atoms must be linked together. However, they don't bond directly to each other. Instead, they share an oxygen atom, creating an ether-like bridge. This gives us our foundational **$P-O-P$ linkage**.

Now, phosphorus is a Group 15 element. In its oxoacids, it typically exhibits a valency of 5, utilizing its empty 3d orbitals to expand its octet. To satisfy this pentavalency, each phosphorus atom forms a strong double bond with an oxygen atom ($P=O$).

### Counting the Bonds

With the core framework established, let's systematically count the specific bonds requested in the problem:

1.  **The $P-O-P$ Bonds:** As we established, there is exactly **$1$** central oxygen bridge connecting the two halves of the molecule.
2.  **The $P=O$ Bonds:** Each of the two phosphorus atoms forms one double bond with an oxygen atom to satisfy its valency. Therefore, we have exactly **$2$** $P=O$ bonds.
3.  **The $P-OH$ Bonds:** We started with 4 hydrogen atoms and 7 oxygen atoms. We've used 3 oxygen atoms so far (one in the bridge, two in the double bonds). The remaining 4 oxygen atoms and 4 hydrogen atoms pair up to form 4 hydroxyl ($-OH$) groups. Each phosphorus atom gets two of these groups. Thus, there are **$4$** $P-OH$ bonds.

### Final Calculation

The question asks for the number of $P-OH$, $P=O$, and $P-O-P$ bonds **respectively**.

Following our structural analysis, the sequence is **$4, 2, 	ext{ and } 1$**. This perfectly aligns with option (d). Always be careful to read the exact order requested in the question to avoid silly mistakes on such scoring problems!

Animated Solution for Chemistry - s and p-Block Elements: In a molecule of pyrophosphoric acid, the number of $P—OH$ , $P = O$ and $P—O—P$ bonds/moiety(ies) respectively are

Select Answer:

Visualized Solution

\text{Structure of Pyrophosphoric Acid}

\text{Identifying } \text{P—O—P} \text{ Bonds}

\text{Count of } \text{P—O—P} \text{ Bonds}

\text{Identifying } \text{P}=\text{O} \text{ Bonds}

\text{Identifying } \text{P—OH} \text{ Bonds}

\text{Final Conclusion}

The Sigma Insight: Group 15 Elements

The "Pyro" Connection

Decoding the Structure

Counting the Bonds

Final Calculation

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Animated Solution for Chemistry - s and p-Block Elements: .math-text-wrapper .katex { font-size: 1.05em !important; } .math-text-wrapper .katex-display { margin: 1rem 0 !important; } In a molecule of pyrophosphoric acid, the number of P—OH, P=O and P—O—P bonds/moiety(ies) respectively are