Question

The protective power of the lyophilic colloids is expressed in terms of gold number a term introduced by Zsigmondy. Gold number is the number of milligram of the protective colloid which prevents the coagulation of $10\text{mL}$ of red gold sol. When $1\text{mL}$ of a $10\%$ solution of sodium chlorideis added to it. Thus, smaller the gold number of lyophilic colloid, the greater is the protective power.

On addition of one mL of solution of $10\%NaCl$ to $10\text{mL}$ of red gold sol in presence of $0.025\text{g}$ of starch, the coagulation is just prevented. The gold number of starch is:

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

Answer 1

The correct option is D $25$

Gold number is the number of milligram of the protective colloid which prevents the coagulation of $10\text{mL}$ of red gold sol. When $1\text{mL}$ of a $10\%$ solution of sodium chlorideis added to it.

$\text{Protective colloid}=0.025\text{g}\to 1\text{mL of}10\%NaCl+10\text{mL gold sol}$
According to the definition of gold number:
$\text{Gold number}=0.025\times 1000\text{mg}=25\text{mg}$