Which of the following is a weak acid?

The dissociation constants of two acids $HA_1$ and $HA_2$ are $3.14 \times 10^{-4}$ and $1.96 \times 10^{-5}$ respectively. The relative strength of the acids will be approximately:

$Cl^{-}$ is the conjugate base of

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 $(HCN)$: $4.9 \times 10^{-10}$

$CH_3COOH_{(aq)} + H_2O_{(l)} \rightleftharpoons H_3O^{+}_{(aq)} + CH_3COO^{-}_{(aq)}$. Between $H_2O$ and $CH_3COO^{-}$,which one is the weaker base?

In the reaction $HBr + H_2O \rightleftharpoons H_3O^{+} + Br^{-}$,the conjugate base of the weak acid is .......