Key Idea: Strength of oxyacids increases with oxidation state of central atom. Find the oxidation state of chlorine in all the oxyacids to decide the decreasing acidic strength among oxyacids. (i) HClO4+1+x+(−2×4)=0∴x=+7 (ii) HClO3+1+x+(−2×3)=0∴x=+5 (iii) HClO2+1+x(−2×2)=0∴x=+3 (iv) HClO+1+x+(−2×1)=0∴x=+1∴HClO4 is strongest acid and HClO is weakest acid. ∴ Correct order of acidic strength is HClO4>HClO3>HClO2>HClO