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CHAPTER 11

Molarity

11.1. INTRODUCTION
Many quantitative chemical reactions are carried out in solution because volumes are easier to measure than
masses. If you dissolve a certain mass of a substance in a measured volume of solution, you can take measured
fractions of the total volume of the solution and know how much of the substance it contains.
Solute is defined as the material that is dissolved in a solvent. For example, if you dissolve sugar in water,
the sugar is the solute and the water is the solvent. In the discussions of this chapter, the solvent will be water;
that is, we will discuss aqueous solutions only.
Perhaps the most useful measure of concentration is molarity. Molarity is defined as the number of moles
of solute per liter of solution:
number of moles of solute
Molarity =
liter of solution
Note particularly that liters of solution are used, not liters of solvent. Chemists often abbreviate the definition
to merely “moles per liter,” but the shortening does not change the way the unit is actually defined. The unit of
molarity is molar, symbolized M. Special care must be taken with abbreviations in this chapter: M stands for
molar; mol for mole(s). (Some authors use M to stand for molarity as well as molar. They may use italics for one
and not the other. Check your text and follow its conventions.)

EXAMPLE 11.1. What is the molarity of the solution produced by dissolving 6.00 mol of solute in enough water to make
1.50 L of solution?
6.00 mol
Ans. Molarity = = 4.00 M
1.50 L
The answer is stated out loud as “The molarity is 4.00 molar,” or more commonly, “The solution is 4.00 molar.” Note
the difference between the words molarity (the quantity) and molar (the unit of molarity).

11.2. MOLARITY CALCULATIONS
Molarity, being a ratio, can be used as a factor. Anywhere M (for molar) is used, it can be replaced by mol/L.
The reciprocal of molarity can also be used.

EXAMPLE 11.2. How many moles of solute are contained in 3.0 L of 2.0 M solution?
Ans. As usual, the quantity given is put down first and multiplied by a ratio—in this case, the molarity. It is easy to
visualize the solution, pictured in Fig. 11-1.
2.0 mol

3.0L = 6.0 mol
1L
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