The coagulation of 100ml of colloidal solution of gold is completely prevented by addition of 0.25g of a substance of X to it before adding 1ml of 10% NaCl solution. the gold number of "X" is-
250 mg of X prevents coagulation of 100 ml of colloidal sol of gold. By definition gold number of X is taht amount of it in mg which prevents coagulation of 10 ml of colloidal gold sol. Hence, to prevent coagulation of 10ml of gold sol, the amount of X required =25 mg, which is the gold number of X.