Molarity vs. Molality — What's the Difference?

Molarity, denoted as M, is defined as the number of moles of solute divided by the total volume of the solution in liters. It is sensitive to temperature changes as volume expands or contracts with temperature. Whereas molality, denoted as m, is defined as the number of moles of solute per kilogram of solvent and remains unaffected by temperature since mass does not change with temperature.

In laboratory settings, molarity is often preferred for reactions that occur in solutions where volume measurement is convenient. On the other hand, molality is used when dealing with high temperatures or conditions where the temperature fluctuates, because its value remains constant regardless of these changes.

Molarity is more commonly used in chemistry for calculating stoichiometric relationships in reactions that take place in aqueous solutions. While molality is particularly useful in studying properties like boiling point elevation and freezing point depression which depend on the amount of solute in relation to the mass of the solvent.

When preparing solutions, molarity is straightforward to calculate when the volume of the solution is known and conditions are controlled. Conversely, molality requires the accurate measurement of the solvent's mass, which can be advantageous when precise solvent ratios are critical, such as in calorimetry.

Molarity can be influenced by the dilution or concentration of the solution since it is dependent on the volume. In contrast, molality remains unchanged during dilution or concentration processes as long as the mass of the solvent and the number of moles of solute remain constant.