The coagulation of 100 mL of gold sol is prevented by addition of 0.25 g of starch to it before adding 1 ml of 10% NaCl solution. If the gold number of starch is x then x5 is:
100 ml of gold sol needs = 250 mg of starch
10 ml of gold sol needs =25010=25 mg
So, x =25
x5=5