Dilutions

  It is evident that when two solutions are mixed, the concentration (measured in mol/L) changes. In the process of dilution, the concentration is decreased by adding a solvent, or any substance that dissolves another substance (usually water.) Similarly to how one adds one cup of water to one can of juice concentrate, or how one adds one cup of water to one can of soup, the substance that is dissolved (solute) does not change in amount. This could be explained with the equation:

n1 = n2    ->    c1v1 = c2v2

 Where c=Concentration, n=number of moles, & v=volume.

EX.1:

Determine C when 200 mL of 0.30 M of Sodium Nitrate (NaNO3) is diluted to a final volume of 600 mL.

1. Remember to first convert mL to L as M is measured in mol/L. After doing so, write out your equation that explains the conservation of the solute before and after dilution.

200 mL  x  1 L    =    0.200 L                     600 mL  x  1 L    =    0.600 L

              1000 mL                                                  1000 mL

c1v1 = c2v2

2. With the information you are given in the question, plug in the numbers into your equation and isolate the variably you are looking for.

(0.30)(0.200)  =  C2(0.600)

                                                                    (0.600)           (0.600)

3. Once you have isolated your variable, calculate & answer the question with correct amount of significant digits.

(0.0600)  =  C2

                                                                                                (0.600)

                                                                       0.10 mol/L = C2

In order to answer other questions similar to this one, you have to consider what is it that the question is asking you to find? In most cases you will have to isolate the variable of which you are solving for.

For more examples on this topic, check out these links:
http://misterguch.brinkster.net/PRA025.pdf

http://www.youtube.com/watch?v=ygr0CGdkAws