The coagulation of 100 mL of a colloidal sol of gold is completely prevented by addition of 0.25 g of starch to it before adding 10 mL of 10% NaCl solution. What is the gold number of starch?

25

The correct answer is option 2. 25.

Starch added to 100 mL of gold sol to completely prevent coagulation by 10 mL of 10% NaCl solution = 0.25 g = 250 mg

This is 10 times the amount required for coagulation of 10 mL of gold sol by 1 mL of 10% NaCl solution

Starch required to be added to 10 mL of gold sol to completely prevent coagulation by 1 mL of 10% NaCl sol = 25 mg

∴ Gold number of starch = 25