This is a dilution problem.
In dilution, the number of moles before dilution and after dilution remains same.
Therefore, we use:
\[
M_1V_1=M_2V_2.
\]
Here,
\[
M_1=0.1M,
\]
\[
V_1=x\text{ ml},
\]
\[
M_2=0.01M,
\]
and
\[
V_2=250\text{ ml}.
\]
Substitute in the formula:
\[
0.1\times x=0.01\times 250.
\]
\[
0.1x=2.5.
\]
Now divide both sides by \(0.1\):
\[
x=\frac{2.5}{0.1}.
\]
\[
x=25.
\]
Hence, the required volume is:
\[
25\text{ ml}.
\]