Find the Gold Number if 700 ml of standard gold solution is just prevented from coagulation by adding 0.35 g of starch before adding 1 ml of 10% NaCl solution?

We know that Gold number is the minimum amount of protective (lyophilic) colloid in milligrams which prevents the  coagulation or flocculation of 10 ml of standard hydro gold sol by the addition of 1 ml of 10% NaCl solution. 

The concept of gold number was given by Zsigmondy.

As par Questions 700 ml of gold solution requires (0.35gm ) or 350 mg of starch.

Hence the amount of lyophilic colloid in mg to protect 10 ml of gold solution is  starch $=(\frac{350\times \mathrm{10)÷}}{700}=\mathrm{5\; mg}$

$\mathrm{"Gold\; number\; is\; 5"}$