The coagulation of `100mL` of a colloidal solution of gold is completely prevented by adding `0.25g` of starch to it before adding `10mL` of `10%NaCl` solution. The gold number of starch is

Options

A. `25`
B. `2.5`
C. `250`
D. `0.25`

Answer

A

Explanation

To calculate gold number, we must take `10mL` of gold sol and add `1mL` of a `10%` solution of `NaCl` to it. Finally we must express the answer in milligrams.
Here `10mL` of `10%NaCl` is added to `100mL` of gold sol. Thus, `1mL` of `10%NaCl` solution is added to `10mL` of gold sol. `100mL` of `10%NaCl` solution is added to `10mL` of gold sol. `100mL` gold sol requires `0.25g` starch or `0.25xx10^(3)mg` starch. Thus, `10mL` gold sols will require.
`(0.25xx10^(3))/(100)xx10mg=25mg`
Thus, by definition, the gold number of starch is `25` .