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

What is the aqueous solution of $HCl$ known as?

Given:
$(i) \ HCN_{(aq)} + H_2O_{(l)} \rightleftharpoons H_3O^{+}_{(aq)} + CN^{-}_{(aq)}$
$K_a = 6.2 \times 10^{-10}$
$(ii) \ CN^{-}_{(aq)} + H_2O_{(l)} \rightleftharpoons HCN_{(aq)} + OH^{-}_{(aq)}$
$K_b = 1.6 \times 10^{-5}$
These equilibria show the following order of the relative base strength:

The conjugate acid of $H_2PO_4^-$ is

For any diprotic acid of the type $H_2A$,the relationship between the first $(Ka_1)$ and second $(Ka_2)$ ionization constants is:

Would gaseous $HCl$ be considered as an Arrhenius acid?