We may apply a mass balance here. The total moles of NaCl will remain constant before and after the mixing of the solutions. We have three solutions: 1) 25 ml of 5 M NaCl 2) 100 ml of 0 M NaCl 3) A mixture of these solutions with volume 125 ml and concentration C
The moles of a substance are equal to the product of the concentration and volume. Thus: