Assertion : $\mathrm{HClO}_4$ is a stronger acid than $\mathrm{HClO}_3$. Reason : Oxidation state of $\mathrm{Cl}$ in $\mathrm{HClO}_4$ is +8 and in $\mathrm{HClO}_3+5$.

Greater the number of negative atoms present in the oxy-acid make the acid stronger. In general, the strengths of acids that have general formula, $(\mathrm{HO})_m \mathrm{ZO}_n$ can be related to the value of $n$. As the value of $n$ increases, acidic character also increases. The negative atoms draw electrons away from the Z-atom and make it more positive. The Zatom, therefore, becomes more effective in withdrawing electron density away from the oxygen atom that bonded to hydrogen. In turn, the electrons of $\mathrm{H}-\mathrm{O}$ bond are drawn more strongly away from the $H$-atom. The net effect makes it easier from the proton release and increases the acid strength.