Question

On addition of 0.5 mL of solution of 10% NaCl to 20 mL of red gold sol in presence of 0.025 g of starch, the coagulation is just prevented. The gold number of starch is

• A. 0.025
• B. 0.1
• C. 2.5
• D. 1

Answer 1

The correct option is B

0.1

Minimum mass (mg) of lyophilic sol to be added to 10 ml of red gold sol which can prevent from coagulation on addition upon 1 ml of 10% NaCl solution is called gold number.

For 20 ml of red gold sol -------- 0.5 ml of 10% NaCl required

For 10 ml of red gold sol -------- $0.5\times \frac{10}{20}$ = 0.25 ml is required

For 0.25 ml of 10% NaCl-------- 0.025 g of starch is required

For 1 ml of 10% NaCl----------- $0.025\times \frac{1}{0.25}$ = 0.1 mg starch is required (gold number)