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Quantitative Methods: Part 1. Solutions and Dilutions

Author(s): David R. Caprette, PhD

Prepare a Solution in Moles/Liter

The first step in the preparation of any solution is to decide how much to make. To simplify the calculations, express the desired volume in liters. For example, if you want to prepare 200 milliliters of a solution, express the volume as 0.2 liter.

Your formula may describe the concentration in moles per liter. For example, 0.25M means 0.25 moles/liter. It may call for a millimolar (mM) solution, a micromolar (µM) solution,a nanomolar (nM) solution, or even a picomolar (pM) solution.  One mM, one µm, one nM, and one pM are 1/1000, 1/1,000,000, 1/1,000,000,000, and 1/1,000,000,000,000 mole/liter, respectively. Whatever prefix is used to describe the desired molar concentration, it is easier to do the calculation by converting the concentration to moles/liter. For example, 25 µM can be expressed as 0.000025M, or 2.5 x 10-5M.

If you know the desired concentration in moles/liter and the formula weight of the substance, you can multiply these numbers and obtain mass per liter (moles/liter X grams/mole = grams/liter). And if you know the volume you need in liters, you then can multiply mass per liter times the desired volume to figure the amount to weigh (grams/liter X volume in liters = grams to weigh).

Remember that extreme precision is seldom either necessary or practical. It usually is good enough to get the actual concentration to within 1% of the desired concentration.