Question

You have a solution of 0.250 M NaCl. If you boil away 2/3 of the volume of water, what is the concentration of this solution?

Answer 1

It makes it easier if we assume a starting volume and I'll show it makes no difference what volume you choose initially. Let's take a volume of 180 mL initially. If we boil away 2/3 of that we are left with 1/3 of 180 mL or 60 mL so the new concentration is 0.250 M x 180/60 = 0.250 x 3 = 0.750 M.
Suppose we chose a volume of 240 mL and boil away 2/3 we are left with 80 mL so the final concentration will be 0.250 x 240/80 = 0.250 x 3 = 0.750 M