Which of the following is a weak acid?

The conjugate acid of $S_2O_8^{2-}$ is

Weak acids are partially ionised in aqueous solutions. The ionisation constants of some acids are given below. Arrange these acids in increasing order of their acid strength.

Acid Index	Acid Name and Ionisation Constant $(K_a)$
$1$	Formic acid $(HCOOH)$: $1.8 \times 10^{-4}$
$2$	Hypochlorous acid $(HClO)$: $3.0 \times 10^{-8}$
$3$	Nitrous acid $(HNO_2)$: $4.5 \times 10^{-4}$
$4$	Hydrocyanic acid $(HCN)$: $4.9 \times 10^{-10}$

Identify each species in the following equilibrium according to the code: $SA = \text{stronger acid}$; $SB = \text{stronger base}$; $WA = \text{weaker acid}$; $WB = \text{weaker base}$. The $pK_a$ of $(CH_3)_2NH_2^+$ is $10.7$ (conjugate acid of $(CH_3)_2NH$) and the $pK_a$ of $CH_3OH$ is $15.2$.
$CH_3OH$ $(1)$ + $(CH_3)_2NH$ $(2)$ $\rightleftarrows CH_3O^- + (CH_3)_2NH_2^+$

Identify the conjugate acid and conjugate base for the $HCO_3{ }^{-}$ ion,respectively.

Which one of the following is not a base?