53. Solving a chemistry problem

822. Rapid coagulation of 75 ml of aluminum oxide sol (whose charge is positive) is observed when 4.11 ml of a 1 mol/l KCl solution is added to it. Based on the DLVO theory, estimate the minimum concentration of the Ca(NO3)2 solution so that 1 ml of its solution has a similar effect on the same volume of the AI2O3 sol.