$(a)\;10\qquad(b)\;1.0\qquad(c)\;0.1\qquad(d)\;0.01$

Answer : $\;0.01$

Explanation :

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 $\;(0r\;1\;cm^{3})\;$ 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 $\;=\large\frac{10^{-4}\times10}{100}=10^{-5}\;g=10^{-2}\;mg$

Therefore , Gold number = 0.01

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