100. Solving a chemistry problem

819. Rapid coagulation of 50 ml of iron hydroxide sol (whose charge is positive) is observed when 1.55 ml of a 0.4 mol/l KNO3 solution is added to it. Based on the DLVO theory, estimate what the minimum concentration of the BaCl2 solution should be in order for 1 ml of its solution to have a similar effect on the same volume of the Fe(OH)3 sol.