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Quantitative Methods: Part 1. Solutions and Dilutions

Author(s): David R. Caprette, PhD

Molarity

A disadvantage of describing formulas as w/v (%) is that the description says nothing about the actual concentration of molecules in solution. What if we want equal amounts of two chemicals to be mixed together, so that for each molecule of substance #1 there is a single molecule of substance #2? The same amount in grams probably will not contain the same number of molecules of each substance.

Another disadvantage of the w/v method is that the same chemical can come in many forms, and one gram of a chemical in one form may contain a different amount of the chemical than one gram in another form. Imagine working with a chemical that can be used in several forms of hydration. For example, calcium chloride (CaCl2), can be purchased as a dry chemical in anhydrous form, so that what you weigh out is nearly all pure calcium chloride. On the other hand, you may have a stock of dry chemical that is hydrated with seven water molecules per molecule of CaCl2. The same mass of this chemical in this form will contain fewer molecules of calcium chloride than in anhydrous form.

Since we frequently want to know the actual concentration of molecules of a chemical in solution, it often is better to have a universal measurement that works regardless of how the chemical is supplied. As long as the molecular weight (sometimes called formula weight) is known, we can describe a solution in the form of moles per liter, or simply molar (M).