Concentration, Dilution, & Stoichiometry

The properties and behavior of many solutions depend not only on the nature of the solute and solvent but also on the concentration of the solute in the solution. Chemists use many different units when expressing concentration; however, one of the most common units is molarity. Molarity (M) is the concentration of a solution expressed as the number of moles of solute per liter of solution:

For example, a 0.25 M NaOH solution (this is read as 0.25 molar) contains 0.25 moles of sodium hydroxide in every liter of solution. Anytime you see the abbreviation M you should immediately think of it as mol/L.

In order to calculate the molarity of a solution, you need to know the number of moles of solute and the total volume of the solution.

To calculate molarity:

1. Calculate the number of moles of solute present.
2. Calculate the number of liters of solution present.
3. Divide the number of moles of solute by the number of liters of solution.

Instead of calculating the moles of solute and liters of solution present individually, you can also string all the calculations together in one problem:

1. Divide the amount of solute by the volume of solution (regardless of the initial units given).
2. Use dimensional analysis to convert the amount of solute to moles of solute.
3. Use dimensional analysis to convert the volume of solution to liters of solution (if necessary).

The following example will illustrate both methods.

Example: What is the molarity of a solution prepared by dissolving 15.0 g of sodium hydroxide in enough water to make a total of 225 mL of solution?

Method 1:

• Calculate the number of moles of solute present.

Serial dilutions