Which acid is stronger or Which is most acidic `:`

The oxide which is most acidic is :

Which is most acidic in nature?

Which is most acidic in nature ?

Justify : Trichloroacetic acid is a stronger acid than dichloroacetic acid (ii) Formic acid is a stronger acid than acetic acid.

It was experimentally found that formic acid is stronger acid than acetic acid. Which of the following effects is useful to explain this?