Predict the pKa value for H2SO4, HSO4 – and SO4 2–.
The Correct Answer and Explanation is:
Correct Answer:
- H₂SO₄ (sulfuric acid): pKa₁ ≈ –3
- HSO₄⁻ (hydrogen sulfate ion): pKa₂ ≈ 1.99
- SO₄²⁻ (sulfate ion): Does not have a relevant pKa (very weakly basic, conjugate base of a strong acid)
Explanation (300+ words):
Sulfuric acid (H₂SO₄) is a diprotic strong acid, meaning it can donate two protons (H⁺) in solution. The acidity of a molecule is quantified using pKa, the negative base-10 logarithm of the acid dissociation constant (Ka). Lower pKa values indicate stronger acids.
- First dissociation: $$
\text{H₂SO₄} \rightarrow \text{H⁺} + \text{HSO₄⁻}
$$ The first proton of H₂SO₄ dissociates completely in aqueous solution. This is why H₂SO₄ is classified as a strong acid. Since this dissociation is virtually 100%, the corresponding pKa is very low, typically estimated to be around –3. Such a low pKa indicates that H₂SO₄ readily donates its first proton, leaving behind the hydrogen sulfate ion (HSO₄⁻). - Second dissociation: $$
\text{HSO₄⁻} \rightleftharpoons \text{H⁺} + \text{SO₄²⁻}
$$ The second proton is not lost as easily, making HSO₄⁻ a weak acid. Its pKa value is approximately 1.99, meaning it only partially dissociates in solution. This value indicates that HSO₄⁻ is still acidic, but significantly weaker than H₂SO₄ itself. - SO₄²⁻ (sulfate ion):
The sulfate ion is the conjugate base of HSO₄⁻. Since HSO₄⁻ has a relatively low pKa (~2), its conjugate base (SO₄²⁻) is very weakly basic and does not act as an acid under normal conditions. Therefore, it does not have a meaningful pKa, as it is not likely to donate a proton further.
Summary:
- H₂SO₄: pKa₁ ≈ –3 (strong acid)
- HSO₄⁻: pKa₂ ≈ 1.99 (weak acid)
- SO₄²⁻: No relevant pKa (conjugate base of a weak aci
