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Quantitative Methods: Part 1. Solutions and Dilutions

Author(s): David R. Caprette, PhD

Example: Prepare 200 ml of 70 mM Sucrose

Suppose you need 200 milliters of a 70 mM solution of sucrose. Two hundred milliliters is 0.2L and 70 mM is 0.07M. The molecular weight of sucrose can be determined from its chemical formula, namely C12H22O11, and the atomic weights of carbon, hydrogen, and oxygen. The formula weight for sucrose is identical to its molecular weight, namely 342.3 grams per mole. A 1M solution would consist of 342.3 grams sucrose in one liter final volume.

A concentration of 70 mM is the same as 0.07 moles per liter. Multiply 0.07 moles/liter by 342.3 grams per mole and you get 23.96 grams needed per liter.  To make 200 milliliters of your solution, multiply grams/liter by liters needed. Since 200 milliliters is 0.2L, multiply 23.96 grams by 0.2L to get the 4.792 grams needed. Since a typical top loading electronic balance displays mass to the nearest 0.01 gram, the amount to be weighed should be rounded to 4.79 grams, although it is perfectly acceptable and perhaps even preferable to round to 4.8 grams.