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# The coagulation of $10 cm^3$ of Gold sol is completely preventred by addition of 0.025g of starch to it. The gold number of starch is

(a) 0.025

(b) 0.25

(c) 2.5

(d) 25

Gold number is the weight of protective colloid in milligrams which prevents the coagulation of $10 cm^{-3}$ of gold sol.
$\therefore$ Gold number of starch = 0.025 $\times$ 1000 = 25 (mg)