you have 505mL of a .125 M HCl solution and you want to dilute it to exactly .100. How much water should you add?

Question

I got (.125M)(505mL)=(.100M)(Vf)=
631.25. But on my answer sheet they got 126 mL of H2O???? I don't know how she got it.

Answer 1

The volume you calculated, 631.25 mL, is the total volume. The question was how much water should be added and that is
631-505 = 126 mL.
(I should point out something. Technically total volume should be 631.25 AND 126 added to 505 will not give EXACTLY 631 mL. because volumes are not EXACTLY additive. Probably I would not have mentioned this if the word exactly had not been present). The point I'm making is that solutions should be made to a specified volume and that is usually done with a volumetric flask. Adding a certain volume of water is not proper and I am not familiar with flasks that can be made to an exact volume of 631 (or 631.25).