Which of the following is not a Lewis acid?

The ionic bond is absent in

Which of the following is the correct order of increasing $O-O$ bond length in the given compounds?

The number of $\pi-$bonds present in a benzoic acid molecule is:

Ionic and covalent bonds are present in

Compare the $x$ and $y$ bond lengths in the following molecules:
Structure $1$: $HO-S(=O)-S(=O)-OH$ (where $x$ is the $S-S$ bond length)
Structure $2$: $HO-S(=O)_2-S(=O)-OH$ (where $y$ is the $S-S$ bond length)