The $pK_a$ value of the strongest acid among the following 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 $(HCN)$: $4.9 \times 10^{-10}$

In the equilibrium $CH_3COOH + HF \rightleftharpoons CH_3COOH_2^+ + F^{-}$,which of the following statements is correct?

Ionisation of which of the following will be maximum in water?

Which one of the following is the most basic?

The conjugate base of $HCO_3^-$ is