Question

$10^{-4}$ g of gelatin is required to be added to $100 cm^3$ of a standard gold sol to just prevent its coagulation by the addition of $1 cm^3 of 10$ solution to it. Hence, the gold number of gelatin is

• A. 10
• B. 1
• C. 0.1
• D. 0.01

Answer 1

Answer: (D)

0.01

Answer 2

By definition, gold number is the number of milligrams of dry protective colloids required to just prevent the coagulation of $10\,mL$ of red gold sol when $1 mL$ (or $1 \,cm^{3})$ of $10\%$ $NaCl$ solution is added to it Here the amount of gelatin required to protect $10 \,mL$ (or $10\, cm^{3})$ of gold sol $=\frac{10^{-4}\times10}{100}=10^{-5}\,g$ $10^{-5} \times 10^{3}\, mg = 10^{-2}\, mg$ $= 0.01 \,mg$ $\therefore$ Gold number $= 0.01$