In an experiment, addition of $$4.0\,ml$$  of $$0.005\,M$$  $$BaC{l_2}$$  to $$16.0\,ml$$  of arsenius sulphide sol just causes the complete coagulation in 2 hrs. The flocculating value of the effective ion is

Solution :
$$A{s_2}{S_3}$$  sol is negatively charged owing to preferential adsorption of $${S^{2 - }}\,ions.$$   Cation would be the effective ion in coagulation. Flocculating value = minimum milli $$mol$$  of the effective ion per litre of sol $$ = \frac{{4 \times 0.005 \times {{10}^3}}}{{4 + 16}} = 1.0$$