If the gold number of potato starch is 25, how many grams of it is required to prevent coagulation of 100mL of gold sol on adding 10mL of 10% NaCl solution ?

Amount of starch (in mg) to be added to 10 ml of gold sol to prevent it's coagulation in the presence of 1 ml of 10% NaCl solution is called gold number.

Amount of starch added = 25 mg or 0.025 g

Amount of starch required to prevent coagulation of 100mL of gold sol on adding 10mL of 10% NaCl solution = $10\times 0.025=0.25\mathrm{g}$