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To resist the coagulation of $100 \mathrm{cc}$ gold sol; 1 cC of $10 \% \mathrm{NaCl}$ is added to it in the presence of $10^{-4} \mathrm{~g}$ gelatin. The gold number of gelatin is
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The correct answer is:
0.01
Gold number is the amount (in milligrams) of aprotective colloid, which is just sufficient to prevent the coagulation of $10 \mathrm{~mL}$ of standard hydro gold sol, when $1 \mathrm{~mL}$ of $10 \% \mathrm{NaCl}$ solution is added to it.
$\because 10 \mathrm{cc}$ gold sol $=1 \mathrm{cc}$ of $10 \% \mathrm{NaCl}$
$\therefore 100 \mathrm{~mL}$ of gold $\mathrm{sol}=1 \mathrm{cc}$ of gelatin
Therefore, gold number of gelatin is
$\frac{1 \times 10}{100}=0.01$
$\because 10 \mathrm{cc}$ gold sol $=1 \mathrm{cc}$ of $10 \% \mathrm{NaCl}$
$\therefore 100 \mathrm{~mL}$ of gold $\mathrm{sol}=1 \mathrm{cc}$ of gelatin
Therefore, gold number of gelatin is
$\frac{1 \times 10}{100}=0.01$
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