H2SO4

Meaning: Sulfuric acid can donate two protons. The first step, H₂SO₄ → H⁺ + HSO₄⁻, goes essentially to completion in water, so H₂SO₄ counts among the common strong acids. The product HSO₄⁻ (hydrogen sulfate, or bisulfate) is a weak acid, and its conjugate base SO₄²⁻ (sulfate) is a very weak base.

Equation: HSO₄⁻ ⇌ H⁺ + SO₄²⁻, with Ka2 = [H⁺][SO₄²⁻] / [HSO₄⁻], where each concentration is in mol/L and Ka2 is about 10⁻² at 25 °C.

On the MCAT: Passages use H₂SO₄ in titrations, pH calculations and acid-rain or battery contexts. In a titration with NaOH, 1 mol of H₂SO₄ reacts with 2 mol of OH⁻, so it supplies 2 equivalents. Questions may also ask you to rank acid strength, for example H₂SO₄ stronger than HSO₄⁻, or to name the conjugate base of each form. Solutions of sulfate salts such as Na₂SO₄ are close to neutral, while NaHSO₄ is acidic.

Common confusion: Students assume both protons dissociate fully and set [H⁺] = 2 × [H₂SO₄]. Only the first step is complete. The second is partial, so [H⁺] is greater than the acid concentration but less than twice it. In very dilute solutions the second step goes closer to completion. For a quick estimate with titrant, count 2 equivalents. For an exact pH, treat the second step with Ka2.