Answer
We should add 126 mL of water.
Work Step by Step
1. Find the necessary volume:
$C_1 * V_1 = C_2 * V_2$
$ 0.125* 505= 0.100 * V_2$
$ 63.125 = 0.100 * V_2$
$V_2 = 631$ mL
2. Calculate the volume difference:
631 mL - 505 mL = 126 mL