Which of the following is the weakest base?

In the following reaction $HCO_3^{-} + H_2O \rightleftharpoons CO_3^{2-} + H_3O^{+}$,which two substances are Bronsted bases?

The $pK_a$ of a weak acid is defined as:

$HF + HClO_4 \rightleftharpoons H_2F^{+} + ClO_4^-$ and $H_2O + H_2PO_4^- \rightleftharpoons H_3O^{+} + HPO_4^{2-}$ Select the group of acidic species present on the right side of the equilibrium.

The total number of diprotic acids among the following is:
$H_3PO_4, H_2SO_4, H_3PO_3, H_2CO_3, H_2S_2O_7, H_3BO_3, H_3PO_2, H_2CrO_4, H_2SO_3$

Which one is not an acid salt?