m or M? Difference Between Molarity and Molality

m and M Units of Concentration in Chemistry

Molarity and molality both are units of concentration of a chemical solution.
Molarity and molality both are units of concentration of a chemical solution. Andrew Brookes / Getty Images

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 molar solution or 0.1 molal solution or if there is even a difference?

What m and M Mean in Chemistry

Both m and M are units of the concentration of a chemical solution. The lower case 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).

Upper case 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 and Molarity

Molality (m) = moles solute / kilogram 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 will not 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 are 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 concentration, 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. Specific examples of times you would use molality rather than molarity are when you're calculating boiling point, boiling point elevation, melting point, freezing point depression, or working with other colligative properties of matter.

Calculate Molarity & Molality | Example Molarity Problems