Question

On addition of 0.8 mL of solution of 10% NaCl to 30 mL of red gold sol in presence of 0.03 g of starch, the coagulation is just prevented. The gold number of starch is

• A. 0.09
• B. 0.4
• C. 0.0375
• D. 5

Answer 1

The correct option is C

0.0375

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 30 ml of red gold sol -------- 0.8 ml of 10% NaCl required

For 10 ml of red gold sol -------- $0.8\times \frac{10}{30}$ = 0.266 ml is required

For 0.8 ml of 10% NaCl-------- 0.03 g of starch is required

For 1 ml of 10% NaCl----------- 0.03x 1/0.8 = 0.0375 mg starch is required (gold number)