Oxygen is more electronegative than chlorine, therefore dispersal of negative charge present on chlorine increases from ClO– to ClO4 – ion because number of oxygen atoms attached to chlorine is increasing. Therefore, stability of ions will increase in the order given below : ClO– < ClO2 – < ClO3 – < ClO4 – Thus due to increase in stability of conjugate base, acidic strength of corresponding acid increases in the following order HClO < HClO2 < HClO3 < HClO4