Question

On addition of 1 mL of 10% NaCl solution to 10 mL gold sol in the presence of 0.025 g of starch, the coagulation is just prevented. Starch has gold number

• A. $2.5$
• B. $25$
• C. $0.25$
• D. $0.025$

Answer 1

Answer: (B)

$25$

Answer 2

Gold Number is the minimum amount of lyophilic colloide in miligrams (starch in given case) which prevents the flocculation of 10 ml gold sol by the addition of 1ml of 10% NaCl solution. $\therefore$ Gold number = miligram of starch $= 0.025 \times 10^3\, mg$ $=25$