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

Gold number of gum arabic = 0.15. 

i.e., 0.15mg of gum arabic is added to 10ml of  gold sol, which prevents the coagulation of gold on adding 1 ml of 10% NaCl solution. 

Now, The amount of gum arabic required to protect 100mL of red gold sol from coagulation  by 10mL of 10% NaCl solution is 

$=\frac{0.15}{10}\times 100=1.5\mathrm{mg}$