please tell me the reason.

H₂SO₄ is a strong base in aqueous solution.

but act as a base in HF Solution.

­­­­­H₂SO₄ + HF­­­ H₃SO₄⁺ + F^-

Although HF is a weak acid in aqueous solution, as it becomes more and more concentrated (approaching 100% hydrogen fluoride) , it becomes a lot more acidic, dissociating into H+ and HF2- ions instead, and doing so far more readily than when it is diluted in water. This could mean that it can donate a proton to the H2SO4, forming the H3SO4+ ion and the stable HF2- ion. Read this (http://en.wikipedia.org/wiki/Hydrofluoric_acid).

Thanks a lot.

it is very useful for me

H2SO4 is a strong acid in aqueous solution, not a base.

H2SO4 is a strong acid in aqueous solution, not a base.

see again my question.

I asked H2SO4 acts as base in HF solution.

according to this equation

H₂SO₄ + HF­­­ H₃SO₄⁺ + F^-

see again my question.

I asked H2SO4 acts as base in HF solution.

according to this equation

H₂SO₄ + HF­­­ H₃SO₄⁺ + F^-

Read what you wrote in the first post.

You said "H2SO4 is a strong base in aqueous solution."

Elementcollector1 was correcting this mistake.

 

I think the answer is that the reaction

H2SO4 + HF­­­ H3SO4+ + F-

does take place, but does not go to completion.