Question

HF is a weaker acid than HCI why?

Answer 1

This question could be answered by getting to know the answer to another question.

Why is F- a stronger base than Cl-?

If you're wondering how the above question's answer answers yours, we know that the stronger an acid is, the weaker its conjugate base is and vice-versa.

Both chloride and fluoride have a unit negative charge on them. The difference is in their sizes. Fluorine is smaller and as a result, the additional electron faces heavy repulsions from the originally existing electrons around it. This destabilizes the ion. Chloride is much larger and can hence efficiently hold a negative charge without as much repulsions.

So now we've established that F- is a stronger base than Cl-.

And as follows, HF is a weaker acid than HCl.