http://www.ck12.org Chapter 16. Solutions
Solution Concentration
In its most general form, concentration describes the number of items in a given area or volume. The units generally
depend on the types of items being counted. For example, if we were to calculate the concentration of people that live
in a city, we would divide the total number of people by the area. If we measured the area in square miles, the units
of concentration for this measurement would be people per square mile. Or, we could determine the concentration
of fish in a lake by dividing the total number of fish by the volume of the lake. If we measure the volume in cubic
meters, the concentration would have units of fish per cubic meter.
To express the concentration of a solution, we can perform a similar calculation. The amount of solute is commonly
measured in terms of moles, but it can also be measured by mass or by total number of particles. We can then divide
this value by either the amount of solvent or the total amount of solution. These values may have units of mass,
volume, moles, or number of particles. Depending on how each component is measured, we get different ways to
measure concentration. In this lesson, we will learn about four different ways to describe the concentration of a
solution:
- Molarity - moles of solute divided by volume (in liters) of solution.
- Molality - moles of solute divided by mass (in kilograms) of solvent.
- Weight percent - mass of solute divided by mass of solution.
- Parts per million (or parts per billion) - particles of solute divided by particles of solution.
Each of these ways of describing concentration will be discussed further below.
Molarity
The most common way to express the concentration of a solution is by determining its molarity. Themolarityof a
solution tells us how many moles of solute are present in each liter of solution. It can be calculated as follows:
Molarity =mol soluteL solution
Molarity has units of moles per liter (mol/L). Moles per liter is also given the abbreviated namemolar(M). For
example, a solution that contains 2 moles of solute in each liter of solution would be a 2 molar (2 M) solution.
Example 16.8
What is the molarity (M) of a 3.4 liter sample of a solution that contains 0.32 moles of NaCl?
Answer:
MNaCl=^0 .32 mol NaCl 3 .4 L = 0.094 M
When indicating solution concentration in molarity, a bracket notation is often used. For instance, from the previous
example we could write:
NaCl =0.094This indicates a 0.094 M solution of sodium chloride.
Making Solutions of a Specific Molarity
To make a solution with a particular concentration of a given solute, the following procedure can be used:
- Calculate the moles of solute that would be present in the entire desired solution, and then use the molar mass
of the solute to calculate the mass that you will need.