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Quantitative Methods: Part 1. Solutions and Dilutions

Author(s): David R. Caprette, PhD

Complications with Formula Weights

Perhaps you cannot find a formula weight on a label, or perhaps you are planning a protocol and do not have the actual chemicals on hand. You can calculate molecular weight from the chemical formula with the aid of a periodic table. You must keep in mind when you purchase the chemical that the formula weight may not be identical to the molecular weight. Suppose you have already determined how much to weigh out, based on the molecular weight, but the formula weight is greater due to hydration or the presence of inert material. Your remedy is simply to multiply your calculated mass by the ratio of formula weight to molecular weight (or simply recalculate the weight needed).

For example, suppose you need 10 grams of pure CaCl2 (m.w. 111.0 g/mol), but discovered that all you have is the hexahydrated form (CaCl2•6H2O, f.w. 219.1 g/mol). Take 219.1 divided by 111.0 and multiply by 10.  You need 19.7 grams of CaCl2•6H2O.

Materials are not always available in 100% pure form. The description on the label might indicate that the chemical is >99% pure. Such is often the case with enzymes or other proteins that must be purified from natural sources. Most of us do not worry about purity if it is above 99%. Greater precision might be important to analytical chemist, for example, but is seldom needed in biological applications. If there are significant impurities or if you insist on being as precise as you can, then calculate the amount of material you need and divide by the fraction representing the purity of the substance. For example, if you need 10 grams of pure substance A but what you have is 95% pure, then divide 10 grams by 0.95 to get 10.5 gram (note that the result has been rounded to a reasonable level of precision).

Most chemicals tend to absorb water unless they are kept desiccated, that is, to some extent they are hygroscopic. This problem should not be confused with the state of hydration, which refers to the direct association of water molecules with molecules of the substance through hydrogen bonding. Magnesium chloride is commonly used in biological buffers, and is notoriously hygroscopic. The formula weight does not include the added mass of water that is absorbed from the atmosphere. In fact, the amount of contamination depends on how long and under what conditions the chemical has been shelved, especially with respect to humidity.

It usually is not practical to worry about water content, since it is so difficult to control. If precision is critical, then chemicals should be maintained under desiccating conditions or used immediately before they can absorb a significant amount of water.