[ZekR]

Question: What volume of 0.0250 M MgCl2 concentrate should be diluted to 250 mL to obtain a solution with [CL-] = 0.0135 M?

I know my logic in solving this is flawed, but I can't put my finger on why. 

In my head, the 250 mL that the concentrate should be diluted to is not relevant - only the volume of 0.0250 M MgCl2 concentrate that contains 0.0135 M of Cl- should be needed.  So I went about it like this:

• L of concentrate contains 0.0250 mol MgCl2 and 0.0500 mol Cl-.
• 1 L of the desired diluted solution contains 0.0135 mol CL-
• 1 L of the concentrate contains 3.7 times the amount of Cl- as needed in the solution: 0.0500 / 0.0135=3.7
• 1 L / 3.7 = 0.27 L, so 270 mL of the concentrate should be needed – but that makes no sense since you cannot dilute 270 mL to 250 mL.

Again, I am obviously missing something obvious here, but please explain what.

Thanks.

[curiouscat]

In my head, the 250 mL that the concentrate should be diluted to is not relevant

That's the problem. It is indeed relevant.

If you wanted to end up with 5 martinis versus just one, would you need the same total amount of Gin?

[Arkcon]

The solution contains 0.0250 of something, you need 0.0135 M of some component of the solution, the second number is much smaller than the first, but you think you can't dilute, because you need more.  Sorry.  You haven't figured this out correctly in bulletin point 3.

[ZekR]

I got it now - thanks!!