As chemists, we often need to dilute solutions from a stock solution.
Although dilution calculations can seem like a bit of a pain at first – once you’ve got them down you’ll be diluting your samples in no time. First of all, consider the C1V1 = C2V2 equation. To be able to use the equation properly you’ll have to know ¾ of the values.
How to use the equation
Consider solution 1 as the sample you want to dilute (or the stock solution) and solution 2 as the sample you want to obtain.
C1 is the concentration of solution 1.
C2 is the concentration of solution 2.
V1 is the concentration of solution 1.
V2 is the concentration of solution 2.
If you know ¾ of the values, you can work out the final one by rearranging the equation. It doesn’t really matter what units are used for C and V as long as they’re consistent, however, it is generally considered good practice to work in molar concentrations and litre volumes. Here’s a worked example that shows how to work with the equation:
A worked example
A chemistry student has a 0.5M stock solution of HCl. However, she needs 50 mL of 0.1M HCl to conduct her experiment. What volume of the stock solution is required for the dilution?
The unknown value here is V1. The equation should be arranged to:
V1 = C2V2/C1
By substituting in the known values from the question the equation becomes:
V1 = (0.1 x 0.05)/0.5 = 0.01L or 10 mL.
So to obtain 50 mL of 0.1M HCl, our student would add 10 mL of 0.5M HCl to a 50 mL volumetric flask and add DI water until the line is reached.
If you struggle with dilution calculations it may be beneficial for you to copy the equation and the worked example into your lab notebook to refer to at a later point. Also, if you are reading through your lab manual in preparation for an experiment and notice that you will need to prepare dilutions it may be helpful to calculate the dilutions before the lab, giving you more time to work on your experiment and discuss any problems with your demonstrators.
Would you like to learn more lab skills? If so, check out our guide to interpreting a flame test.