What volume of 0.149 M HCl must be added to 1.00x10^2 mL of 0.285 M HCl so that the resulting solution has a molarity of 0.205 M? Assume that the volumes are additive.

Answer 1

Let x = volume in mL of 0.149 M HCl to be added and mmols = millimols.
mmols 0.285M + mmols 0.149M = mmols 0.205M
(0.285*100) + (0.149x) = 0.205*(100+x)
Solve for x.
You should prove that by adding mmols 0.285M to mmols 0.149M and see if total mmols/total volume in mL = 0.205M