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Molal Concentration

The concentration of a solution refers to the amount of solute in a given amount of solvent.  There are two imprecise terms used in this connection.  A dilute solution is one that contains relatively little solute in a large amount of solvent.  A concentrated solution contains a relatively large amount of solute in a given amount of solvent.  As you can see these terms are rather subjective, however for commercially available solutions, particularly acids, the term concentrated refers to the maximum concentration available.

There are many ways to express, quantatively, the concentration of a solution.   We have already noted that molarity is perhaps the most popular unit of concentration with chemists, molarity does have one important flaw however. Its value changes with temperature.  If we wish to have a unit of concentration that is independent of temperature, then it must be based on mass not volume.  One such unit is molality, which is the number of moles of solute per kilogram of solvent.  

The molality (m) or molal concentration is the amount of solute, in moles, per kilogram of solvent.

moles of solute

molality (m)

=

---------------------

kilograms of solvent

Lets work through an example problem that will illustrate this concept.

Example problem

What is the molality of a solution in which 3.57 g of sodium chloride, NaCl, is dissolved in 25.0 g of water?

Solution

First, convert from mass of NaCl to moles of NaCl.

1 mole NaCl

3.57 g NaCl

x

---------------------

=

0.0611 mole NaCl

58.44 g NaCl

 

Mentally convert 25.0 g to 0.0250 kg and substitute these two quantities into the defining equation for molality.

 

0.0611 mol NaCl

Molality

=

--------------------

=

2.44 m NaCl

0.0250 kg solvent


We read this as 2.44 molal NaCl.

 

 







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