Gold number of gum arabic is 0.15. The amount of gum arabic required to protect 100mL of red gold sol from coagulation by 10mL of 10% NaCl solution is

$$\begin{gathered} \mathrm{Gold} \mathrm{number} \mathrm{of} \mathrm{Gum} \mathrm{Arabic} =0.15 \\ 0.15 \mathrm{mg} \mathrm{of} \mathrm{Lyophilic} \mathrm{solid} \mathrm{needed} \mathrm{in} 10 \mathrm{ml} \\ 10 \mathrm{ml} —0.15\mathrm{mg} \\ 100\mathrm{ml}—? \\ \frac{100\times 0.15}{10}=1.5\mathrm{mg} \\ 1.5 \mathrm{mg} \mathrm{of} \mathrm{Gold} \mathrm{will} \mathrm{be} \mathrm{needed} \mathrm{to} \mathrm{protect} 100 \mathrm{ml} \mathrm{of} \mathrm{red} \mathrm{color} \end{gathered}$$