You have 1.0 L of a 2.5 M solution of KCL. You need 250 mL of a 0.5 M solution of KCL. Describe how you would go about making this dilution in the labratory.

Question

You have 1.0 L of a 2.5 M solution of KCL.  You need 250 mL of a 0.5 M solution of KCL.  Describe how you would go about making this dilution in the labratory.

mww · Answer

The intuitive way is to think of how many times more dilute your original solution must be. Going from 2.5 M to 0.5 M is a 5 fold dilution. This can be achieved by having the original solution diluted into a final volume five times the original.

So you need 1 part of the original 2.5M solution with 4 parts of water so that final volume is 5 times larger. The final volume specified is 250mL. This is the same as 50:200 as a 1:4 ratio. So take 50mL of your 2.5 M KCl and add 200mL of water to it to a total of 250 mL of 0.5M KCl.

Another way to find out how many moles of KCl you have in your final solution
n = cV = 0.250 L x 0.5 M = 0.125 mol. This number of moles has to come from your original solution. The concentration of the original solution is 2.5 M. 
So volume you are required to dilute from is V = n/c = 0.125 mol/2.5 M = 0.05L 
This is 50 mL of 2.5 M KCl and since the final volume of your dilute solution is 250mL, just add 200 mL to it.