Answer

Analysis of Sulfuric Acid Species at pH 7

Chemical Background

• Sulfuric acid (H₂SO₄) is a strong diprotic acid.
• It dissociates in two steps:
  1. H₂SO₄ → H⁺ + HSO₄⁻   (strong acid, pK_a1 = -6.38)
  2. HSO₄⁻ ⇌ H⁺ + SO₄²⁻   (weak acid, pK_a2 = 2.00)
• At pH 7, both acidic protons have been neutralized by NaOH.

Species Present Above 0.1 mol/L

• [SO₄²⁻] — Final form after complete deprotonation of H₂SO₄. Likely to be > 0.1 M.
• [Na⁺] — Added via NaOH. Matches amount of OH⁻ added to neutralize both protons; present > 0.1 M.

Species Present Below 0.1 mol/L

• [H₂SO₄] — Fully deprotonated at pH 7. Negligible concentration.
• [HSO₄⁻] — pKa = 2. At pH 7, mostly converted to SO₄²⁻.
• [H₃O⁺] — [H⁺] = 10^-7 M at pH 7, much less than 0.1 M.
• [OH⁻] — [OH⁻] = 10^-7 M at pH 7, also negligible.

Conclusion

At pH 7, the major species present above 0.1 mol/L are:

• SO₄²⁻ (fully deprotonated sulfuric acid)
• Na⁺ (from NaOH neutralization)