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Quantitative Methods: Part 1. Solutions and Dilutions

Author(s): David R. Caprette, PhD

Working with Formula Weights

As with w/v solutions, we weigh out a specific amount of chemical when making a molar solution.  Unlike w/v solutions, the amount to weigh depends on the molecular weight (m.w.) of the substance in grams per mole (g/mol). To calculate the desired mass of solute, you will need to know the formula weight. Formula weights usually are printed on the label and identified by the abbreviation f.w. Formula weight is the mass of material in grams that contains one mole of substance. It may include inert materials and/or the mass of water molecules, in the case of hydrated compounds. For pure compounds, the formula weight is the molecular weight of the substance and may be identified as such.

For example, the molecular weight of calcium chloride is 111.0 grams per mole (g/mol), which is the same as the formula weight if the material is anhydrous.  Calcium chloride dihydrate (CaCl2•2H2O) is 147.0 g/mol. The formula weight for CaCl2•6H2O (hexahydrate) is 219.1 g/mol.

A hydrated compound is a compound that is surrounded by water molecules held in place by hydrogen bonds. The water molecules in a hydrated compound become part of the solution when the material is dissolved. Thus, 111.0 grams of anhydrous CaCl2, 147.0 grams of dihydrated CaCl2, or 219.1 grams of CaCl2 hexahydrate in one liter final volume all produce a 1 mole per liter solution, abbreviated 1M.

Suppose you need one liter of a solution of 10 mM calcium chloride (10 millimolar, or 0.01 moles per liter), and you have only CaCl2 dihydrate. To make your 10 mM solution, you would weigh out 1/100 of the formula weight for dihydrated CaCl2, which is 0.01 x 147.0 = 1.47 grams and bring it to one liter.