The conjugate acid of $H_2PO_4^-$ is

Which of the following is known as the hydronium ion?

The strength of an acid depends on its tendency to

Which activity from the following is exhibited by a $Lewis$ base according to its definition?

The number of species among the following that can act both as Bronsted acids and bases is $HCl, ClO_4^{-}, OH^{-}, H^{+}, H_2O, HSO_4^{-}, SO_4^{2-}, H_2SO_4, Cl^{-}$.

$K_{a}$ values for acids $H_{2}SO_{3}$,$HNO_{2}$,$CH_{3}COOH$ and $HCN$ are respectively $1.3 \times 10^{-2}$,$4 \times 10^{-4}$,$1.8 \times 10^{-5}$ and $4 \times 10^{-10}$. Which of the above acids produces a stronger conjugate base in aqueous solution?