The Difference Between Molality and Molarity

If you pick up a stock solution from a shelf in the lab and it's 0.1 m HCl, do you know if that's a 0.1 molal solution or a 0.1 molar solution, or if there is even a difference? Understanding molality and molarity is important in chemistry because these units are among the most commonly used to describe solution concentration.

What m and M Mean in Chemistry

Both m and M are units of the concentration of a chemical solution. The lowercase m indicates molality, which is calculated using moles of solute per kilograms of solvent. A solution using these units is called a molal solution (e.g., 0.1 m NaOH is a 0.1 molal solution of sodium hydroxide). Uppercase M is molarity, which is moles of solute per liter of solution (not solvent). A solution using this unit is termed a molar solution (e.g., 0.1 M NaCl is a 0.1 molar solution of sodium chloride).

Formulas for Molality

Molality (m) = moles solute / kilograms solvent
The units of molality are mol/kg.

Molarity (M) = moles solute / liters solution
The units of molarity are mol/L.

When m and M Are Almost the Same

If your solvent is water at room temperature, m and M can be roughly the same, so if an exact concentration doesn't matter, you can use either solution. The values are closest to each other when the amount of solute is small because molality is for kilograms of solvent, while molarity takes into account the volume of the entire solution. So, if the solute takes up a lot of volume in a solution, m and M won't be as comparable.

This brings up a common mistake people make when preparing molar solutions. It's important to dilute a molar solution to the correct volume rather than add a volume of solvent. For example, if you're making 1 liter of a 1 M NaCl solution, you would first measure one mole of salt, add it to a beaker or volumetric flask, and then dilute the salt with water to reach the 1-liter mark. It is incorrect to mix one mole of salt and one liter of water.

Molality and molarity are not interchangeable at high solute concentrations, in situations where the temperature changes, or when the solvent is not water.

When to Use One Over the Other

Molarity is more common because most solutions are made by measuring solutes by mass and then diluting a solution to the desired concentration with a liquid solvent. For typical lab use, it's easy to make and use a molar concentration. Use molarity for dilute aqueous solutions at a constant temperature.

Molality is used when the solute and solvent interact with each other, when the temperature of the solution will change, when the solution is concentrated, or for a nonaqueous solution. You would also use molality rather than molarity when you're calculating boiling point, boiling point elevation, melting point, or freezing point depression or working with other colligative properties of matter.

Learn More

Now that you understand what molarity and molality are, learn how to calculate them and how to use concentration to determine mass, moles, or volume of the components of a solution.