The dissociation of $H_3PO_4$ takes place in how many steps?

Which one is a Lewis acid?

Given below are two statements,one is labelled as Assertion $A$ and the other is labelled as Reason $R:$
Assertion $A :$ The amphoteric nature of water is explained by using Lewis acid/base concept.
Reason $R :$ Water acts as an acid with $NH_{3}$ and as a base with $H_{2}S$.
In the light of the above statements,choose the correct answer from the options given below:

The conjugate base of $H_2PO_4^-$ is

The one which has the highest value of $pH$ is

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