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Describing Solutions

Author(s): David R. Caprette, PhD

Describing Solutions - Formulas

Suppose that someone already has worked out the details, so all you have to do is read a formula and make a solution. We usually can assume that a solution is to be aqueous unless stated otherwise. What about the concentration of the substance to be added?

Common ways of describing the concentrations of solutions are weight-in-weight, weight-in-volume, volume-in-volume, and molarity. Less commonly used descriptions include normality and molality. We will go over them one at a time in part II, but first let's look at what they all have in common. A quantity of solute is measured out, mixed with solvent, and the volume is brought to some final value after the solute is completely dissolved. That is, solutions are typically prepared volumetrically. Because solutes add volume to a quantity of solvent, this method of preparation of solutions is necessary to ensure that an exact desired concentration is obtained.

There are exceptions, of course. For example, culture media for bacteria typically are made by adding a measured amount of powdered medium to a measured volume of water. In such cases, it isn't critical that a precise concentration be obtained, so a weight-to-volume method is appropriate, instead of weight-in-volume.