Hello!
Sulfuric acid (H₂SO₄) is a strong diprotic acid. "Diprotic" means that the acid donates 2 protons when dissociating in water to form ions. This happens in a series of two steps which are described below. The first reaction is basically a complete reaction while the second one is an equilibrium:
H₂SO₄(aq) + H₂O(l) → HSO₄⁻(aq) + H₃O⁺(aq)
HSO₄⁻(aq) + H₂O(l) ⇄ SO₄⁻²(aq) + H₃O⁺(aq)
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