CH 14 CONVERTING MOLARITY TO MOLALITY

CH 14 CONVERTING MOLARITY TO MOLALITY - C. Converting...

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C. Converting between Molality and Molarity In problems of conversion between molality and molarity you will need to be given the density of the solution. Since this varies with both the identity and the concentration of the solution, it must be given as part of the problem. You will need it to convert from mass in molality to volume in molarity. Note also that not only do the units change, but the material also changes from solution (in molarity) to solvent (in molality). The concentration of a solution does not depend on how much solution is present. You will probably find it convenient to choose a particular amount of solution. The most convenient thing is to choose 1 L of solution if you are starting with molarity and 1 kg of solvent if you are starting with molality. >> Example 5 What is the molality of a 0.123 M HCl( aq ) solution? The density of the solution is 1.030 g/mL. Solution: molality = mol HCl/kg water molarity = mol HCl/L solution Since we have a value of molarity given, let's assume that we are working with exactly 1 L of solution. Using the molarity equation M = mol/L or M L = mol, there is (0.123 M )(1 L) = 0.123 mol HCl. 0.123 mol HCl 36.45 g 1 mol = 0.0448 g HCl Using the density 1 L 1000 mL 1 L 1.030 g 1 mL = 1030 g solution g solvent = g solution – g solute = 1003 g – 0.0448 g = 1030 g solvent 1030 g solvent 1 kg 1 g = 1.030 kg
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Using the molality equation m = mol HCl kg solvent
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This note was uploaded on 05/31/2008 for the course CHM 2045 taught by Professor Mitchell during the Spring '07 term at University of Florida.

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CH 14 CONVERTING MOLARITY TO MOLALITY - C. Converting...

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