K_a1 = 3 × 10⁻²,
K_a2 = 1.7 × 10⁻⁷

Structure and Ionization Constants of Phosphorous Acid (H₃PO₃)

Phosphorous acid (H₃PO₃) is an important oxyacid of phosphorus.
Even though the formula shows three hydrogens, not all hydrogens are ionizable.
The molecular structure explains why it behaves as a diprotic acid and shows only two ionization constants.

Correct Structure of Phosphorous Acid

Structural formula:

        O
        ||
    HO — P — OH
         |
         H

• One P = O double bond
• Two –OH groups
• One P–H bond

Important rule: Only hydrogens attached to oxygen (–OH) are acidic.
The hydrogen directly attached to phosphorus (P–H) is non-ionizable.

Therefore, ionizable hydrogens = 2

Ionization Constants Given

K_a1 = 3 × 10⁻²
K_a2 = 1.7 × 10⁻⁷

Stepwise Ionization Reactions

First Ionization (Strong)

H₃PO₃ ⇌ H⁺ + H₂PO₃⁻

The first –OH hydrogen dissociates easily, so K_a1 is large
(3 × 10⁻²).

Second Ionization (Weak)

H₂PO₃⁻ ⇌ H⁺ + HPO₃²⁻

The second –OH hydrogen dissociates with more difficulty, so K_a2 is smaller
(1.7 × 10⁻⁷).

Third Ionization

Not possible because the P–H hydrogen cannot ionize. Hence, no K_a3.

Explanation of Options (MCQ Analysis)

Option A

Contains two –OH groups, one P–H bond and one P=O bond.
Correctly explains diprotic nature. Correct Answer.

Option B

Shows three –OH groups, which would make the acid triprotic like H₃PO₄. Incorrect.

Option C

Incorrect bonding arrangement and violates phosphorus valency. Incorrect.

Option D

Missing proper –OH groups and does not match given ionization constants. Incorrect.

Quick Exam Trick

Tip: Count only –OH hydrogens to find basicity.

Final Answer Summary

• Structure: HP(O)(OH)₂
• Nature: Diprotic acid
• K_a1 = 3 × 10⁻²
• K_a2 = 1.7 × 10⁻⁷
• Correct Option: A