Introduction

Determining the predominant ionic species of weak polyprotic acids is an important concept
in acid–base chemistry and competitive exams. By comparing pH with pKa values,
we can easily predict which form of the acid dominates in solution.
Here we analyze phosphorous acid using its ionization constants.

Step 1: Calculate pK_a Values

First dissociation:
pK_a1 = −log(3 × 10⁻²) ≈ 1.5

Second dissociation:
pK_a2 = −log(1.7 × 10⁻⁷) ≈ 6.77

Step 2: Compare pH with pK_a

Rule:

• If pH < pK_a → Protonated form dominates
• If pH > pK_a → Deprotonated form dominates

Since 1.5 < 5 < 6.77, the first proton is removed but the second is not removed significantly.
Therefore, the mono-deprotonated species dominates.

Step 3: Dissociation Reactions

H₃PO₃ ⇌ H⁺ + H₂PO₃⁻

H₂PO₃⁻ ⇌ H⁺ + HPO₃²⁻

Explanation of Each Option

(A) H₃PO₃
Exists only when pH < 1.5 → Incorrect

(B) H₂PO₃⁻
Dominates when pK_a1 < pH < pK_a2 → Correct

(C) HPO₃²⁻
Dominates only when pH > 6.77 → Incorrect

(D) PO₃³⁻
Third ionization does not occur significantly → Incorrect

Final Answer

(B) H₂PO₃⁻

Quick Exam Shortcut

Key Takeaways

• Always convert K_a to pK_a
• Compare pH with pK_a
• Middle region gives intermediate species
• For pH 5 → H₂PO₃⁻ dominates