Which of the following is not a Lewis acid?

The strength of an acid depends on its tendency to

Write the conjugate base of the following: $HF, CH_{3}NH_{3}^{+}, H_{3}PO_{4}, HPO_{4}^{2-}$

The species among the following,which can act as an acid and a base is

The dissociation constants of two acids $HA_1$ and $HA_2$ are $3.14 \times 10^{-4}$ and $1.96 \times 10^{-5}$ respectively. The relative strength of the acids will be approximately:

Which of the following pairs of species is used in the preparation of antacid medicines?