How to Calculate Molarity of a Solution

Molarity is a unit of concentration, measuring the number of moles of a solute per liter of solution. The strategy for solving molarity problems is fairly simple. This outlines a straightforward method to calculate the molarity of a solution.

The key to calculating molarity is to remember the units of molarity (M): moles per liter. Find the molarity by calculating the number of moles of the solute dissolved in liters of a solution.

Sample Molarity Calculation

• 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 the moles nor liters needed to find molarity, so you must find the number of moles of the solute first.

To convert grams to moles, the molar mass of the solute is needed, which can be found on certain periodic tables.​

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Molarity Definition in Chemistry

By Anne Marie Helmenstine, Ph.D.

• 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 (moles per liter).

Quick Review of Calculating Molarity

To calculate molarity:

• Find the number of moles of solute dissolved in solution,
• Find the volume of solution in liters, and
• Divide moles solute by liters solution.

Make certain to use the correct number of significant figures when reporting your answer. One easy way to track the number of significant digits is to write all your numbers in scientific notation.