Thanks! I was hoping the answer choices would allow for a "shortcut" to the answer, but that's not the case here. I would guess that TBR's reference to "averaging the solutions" simply means (total moles) / (total volume), as described by Asteri, which is probably the most straightforward way of solving this question.

However, here's a different approach that's a little more conceptual and easier on the math. Maybe it'll be faster for you, maybe not.

The first solution is 0.1 M KCl, which is effectively **0.1 M Cl-**

The second solution is 0.2 M MgCl2, which is effectively **0.4 M Cl-**

The molarity of Cl- of the mixed solution is the average of 0.1 and 0.4 M weighted by their respective volumes (50 and 25 mL). Notice the **2:1 ratio **of these volumes; this is 2 parts 0.1 M and 1 part 0.4 M. This means that the answer will be closer to 0.1 M than it will be to 0.4 M (which unfortunately cancels out only 0.25 M, I was hoping for more!)

If you simply calculate the average, [2(0.1)+(0.4)]/3 = 0.2

If you're *into fractions*, the average is one third the way between 0.1 and 0.4 M, or 0.2 M