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Molar Concentration

Reactions between solids are usually extremely slow, for that reason chemists usually carry out reactions in solution.  A solution is a homogeneous mixture of two or more substances.  The components of a solution are the solute(s), the substances(s) being dissolved, and the solvent, the substance in which the solute(s) are dissolved.  The solutes are usually the components present in smaller amounts, and the solvent is usually present in the greater amount.  

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.  Undoubtedly the concentration unit used most often by chemists is molarity. Two factors contribute to the popularity of molarity with chemists. (1) Substances undergo chemical combinations in certain molar ratios.  (2) Volumes of solutions are easier to measure than masses of solution.

The molarity (M) or molar concentration is the amount of solute, in moles, per liter of solution.

moles of solute

molarity (M)

=

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

liters of solution

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

Example problem

What is the molarity of a solution in which 3.57 g of sodium chloride, NaCl, is dissolved in enough water to make 25.0 ml of solution?

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 mL to 0.0250 L and substitute these two quantities into the defining equation for molarity.

 

0.0611 mol NaCl

Molarity

=

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

=

2.44 M NaCl

0.0250 L solution


We read this as 2.44 molar NaCl.

 It is important to understand that molarity means moles of solute per liter of solution, and not per liter of solvent. 

 







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