Question

HI is stronger acid than HF. explain

Answer 1

I can presume why you are asking this particular question.

See, Chemistry is an experimental science. Here we do have stats first and then provide the reason accordingly.

Here, as suggested by many other answers, one can explain the phenomenon by the remarks:

H-I has longer bond length; long enough for H+ and I- to dissociate. This causes it to be more acidic in nature.
I- has a stable hold with the negative charge due to large size (Delocalization)
(Both these explanations satisfy the point, and so can be regarded correct)

Now consider that if the opposite result had been observed (Ka values), we could have probably explained it as:

Fluorine is more electronegative. So, it pulls the electron towards it more. This causes the H+ ion to be more free, causing greater acidic nature.
You get my point?

Thanks for reading.

Answer 2

HEY!!

Iodine has a greater nuclear charge and so is a electron withdrawing group....that is why HI is stronger acid when compared to HF