Q.1 The pH of a 0.1 M solution of monosodium succinate (pKa1 = 4.19 and pKa2 = 5.57) is ____.
The pH of a 0.1 M monosodium succinate solution is 4.88. Monosodium succinate acts as an ampholyte from the diprotic succinic acid, where the pH approximates the average of the given pKa values using the Henderson-Hasselbalch equation for intermediate forms.
Solution Method
Succinic acid (H₂A) has pKa₁ = 4.19 (H₂A ⇌ HA⁻ + H⁺) and pKa₂ = 5.57 (HA⁻ ⇌ A²⁻ + H⁺). Monosodium succinate provides the monoanion HA⁻ at 0.1 M.
For such amphoteric species where pKa₂ – pKa₁ ≈ 1.38 (not too far apart), both equilibria contribute comparably, yielding pH ≈ (pKa₁ + pKa₂)/2 = (4.19 + 5.57)/2 = 4.88.
This holds because [HA⁻] >> [H⁺, OH⁻, A²⁻, H₂A], making [HA⁻] ≈ 0.1 M and the ratio [A²⁻]/[H₂A] ≈ 1 from K₂K₁ ≈ [H⁺]².
Detailed Calculation
From charge balance: [H⁺] + [Na⁺] = [HA⁻] + 2[A²⁻] + [OH⁻], but [Na⁺] = 0.1 M and minor species are negligible.
Equilibrium gives K₁[H₂A] = [H⁺][HA⁻] and K₂[HA⁻] = [H⁺][A²⁻], so [H⁺]² ≈ K₁K₂, or pH = ½(pKa₁ + pKa₂) = 4.88.
Exact Henderson-Hasselbalch for HA⁻ buffer: pH = pKa₁ + log([HA⁻]/[H₂A]) or pKa₂ + log([A²⁻]/[HA⁻]), converging to the average.
Monosodium succinate, a key buffer in biochemistry, forms a 0.1 M solution with pH determined by its amphoteric nature between pKa1=4.19 and pKa2=5.57. This pH of 0.1 M monosodium succinate solution calculation is vital for CSIR NET aspirants tackling polyprotic acid salts.
Step-by-Step Derivation
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Identify HA⁻ (monosodium succinate) as intermediate form of H₂Succ.
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Apply ampholyte approximation: pH = (pKa1 + pKa2)/2 since ΔpKa < 3.
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Compute: (4.19 + 5.57)/2 = 4.88 (matches exam integer fill).
No options provided, but common traps include:
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Using single pKa (e.g., pH=4.19 or 5.57): Wrong, ignores dual dissociation.
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Treating as strong acid: pH=1, invalid for weak pKa values.
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Henderson for 1:1 buffer: Requires mixing acid/salt, not pure salt.
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Average miscalculation (e.g., 4.9): Precise to 4.88.
CSIR NET Relevance
This question tests polyprotic buffers, appearing in IIT JAM/GATE too. Practice with succinate buffers for pH 4-6 range in enzyme kinetics.
