JEE Main · 2008 · 27 Apr 2008 · Offline · Equilibrium

Ranking by acid strength, fluorosulphuric acid $\mathrm{HS}{\mathrm{O}}_3\mathrm{F}$ is a superacid and the strongest of the four; the hydronium ion ${\mathrm{H}}_3{\mathrm{O}}^{+}$ comes next; the hydrogen sulphate ion $\mathrm{HS}{\mathrm{O}}_4^{-}$ is only moderately acidic; and the bicarbonate ion $\mathrm{HC}{\mathrm{O}}_3^{-}$, which is in fact nearer to basic, is the weakest. In order of increasing strength this reads $\mathrm{HC}{\mathrm{O}}_3^{-}<\mathrm{HS}{\mathrm{O}}_4^{-}<{\mathrm{H}}_3{\mathrm{O}}^{+}<\mathrm{HS}{\mathrm{O}}_3\mathrm{F}$, that is (i) $<$ (iii) $<$ (ii) $<$ (iv).