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On addition of one $\mathrm{ml}$ solution of $10 \% \mathrm{NaCl}$ to $10 \mathrm{~ml}$ gold sol in the presence of $0.25 \mathrm{gm}$ of starch, the coagulation is just prevented. Starch has the following gold number
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250
By definition gold number of starch is the amount of starch in $\mathrm{mg}$ added to 10 $\mathrm{ml}$ standard gold sol which prevents the coagulation of gold on adding $1 \mathrm{~ml}$ of $10 \% \mathrm{NaCl}$ solution. So the amount of starch is $0.25 \mathrm{~g}=250 \mathrm{mg}$. Hence gold number is 250.
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