The key to calculating molarity is to remember the units of molarity: moles per liter. Find the number of moles of the solute dissolved in liters of solution. Take the following example:
Calculate the molarity of a solution prepared by dissolving 23.7 grams of KMnO4 into enough water to make 750 mL of solution.
This example has neither moles or liters needed to find molarity. Find the number of moles of the solute first.
To convert grams to moles, the molar mass of the solute is needed. From the periodic table:
Molar mass of K = 39.1 g
Molar mass of Mn = 54.9 g
Molar mass of O = 16.0 g
Molar mass of KMnO4 = 39.1 g + 54.9 g + (16.0 g x 4)
Molar mass of KMnO4 = 158.0 g
Use this number to convert grams to moles.
moles of KMnO4 = 23.7 g KMnO4 x (1 mol KMnO4/158 grams KMnO4)
moles of KMnO4 = 0.15 moles KMnO4
Now the liters of solution is needed. Keep in mind, this is the total volume of the solution, not the volume of solvent used to dissolve the solute. This example is prepared with 'enough water' to make 750 mL of solution.
Convert 750 mL to liters.
Liters of solution = mL of solution x (1 L/1000 mL)
Liters of solution = 750 mL x (1 L/1000 mL)
Liters of solution = 0.75 L
This is enough to calculate the molarity.
Molarity = moles solute/Liter solution
Molarity = 0.15 moles of KMnO4/0.75 L of solution
Molarity = 0.20 M
The molarity of this solution is 0.20 M.
To calculate molarity
- Find the number of moles of solute dissolved in solution.
- Find the volume of solution in liters.
- Divide moles solute by liters solution.
More Molarity Example Problems
Molarity Worked Example
Molarity and Concentration Example Problem