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Quantitative Methods: Part 2. Diluting Solutions

Author(s): David R. Caprette, PhD

Making Dilutions

Many students panic when they must dilute something, yet the mathematics involve nothing worse than the simplest algebra. What is the objective?  If the idea is to dilute a starting volume of known concentration to some final concentration, then the objective is to determine the volume of the final product. If the idea is to dilute stock solution to some final concentration AND final volume, then it is the starting volume of stock solution that must be determined.

It is easy to work out dilution problems once a solid frame of reference is established. First, consider that the total amount of solute in a solution is equal to its volume times its concentration (e.g., 1/2 liter of 0.2 M sucrose contains 0.1 mole sucrose). Next, consider that if you take a volume of solution of a certain concentration and dilute it, you are adding solvent without changing the total amount of solute. Thus, the starting concentration multiplied by starting volume must equal the final concentration multiplied by the final volume. Once students understand the principle behind the equation C1V1=C2V2, dilution problems should be easy for them.