What is the charge of an oxygen ion? -

The ionic charge of oxygen is -2.

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Answer is: concentration of sulfuric acid is 0.00125 M.

Chemical reaction (dissociation) of sulfuric acid in water:
H₂SO₄(aq) → 2H⁺(aq) + SO₄²⁻(aq).
From chemical reaction: n(H⁺) : n(SO₄²⁻) = 2 : 1.
[H₂SO₄] = [H⁺] ÷ 2.
[H⁺] = 0.0025 M; equilibrium concentration of hydrogen cations.
[H₂SO₄] = 0.0025 M ÷ 2.
[H₂SO₄] = 0.00125 M; equilibrium concentration of sulfuric acid.