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# The coagulation of 100 ml of a colloidal solution of gold is completely prevented by the addition of 0.25 g of starch to it before adding 1 ml of 10% Nacl solution . Calculate the gold number of starch .

$(a)\;20\qquad(b)\;0.25\qquad(c)\;2.5\qquad(d)\;25$

Explanation :
Amount of starch added to 100 mL of gold sol required to prevent coagulation of 1 mL of $\;10\%\;$ Nacl solution = 0.25 g = 250 mg
Starch required to be added to 10 mL to gold sol to completely . Prevent coagulation by 1 mL of $\;10\%\;$ Nacl solution .
$=\large\frac{250}{100}\times 10 = 25 mg$
Therefore , Gold number of starch = 25
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