Why is concentrated sulphuric acid is weak acid

Sulfuric acid is a strong acid in any state

Explanation:

You may be thinking if the definition that a weak acid produces relatively few ions in aqueous solution,

If you have 100 % sulfuric acid, there is no water, so the definition does not apply.

Commercial sulfuric acid contains about 2 % water. The sulfuric acid in that small amount of water consists of about 99 % HSO₄⁻ and 1 % SO₄²⁻. The acid is almost completely ionized.

Sulfuric acid is a strong acid.