Molarity vs Molality: What's the Difference?
Molarity (M) and molality (m) are two ways to express concentration. While they sound similar, they differ in a crucial way: molarity depends on volume, while molality depends on mass.
Molarity = moles of solute / liters of solution (mol/L). Molality = moles of solute / kg of solvent (mol/kg).
The key difference: molarity changes with temperature (because volume expands/contracts), while molality stays constant (mass doesn't change with temperature).
When to use molarity: Most lab work, titrations, solution preparation. When to use molality: Boiling point elevation, freezing point depression, colligative properties.
Example: 1 mol NaCl in 1 L water = 1 M (molarity). 1 mol NaCl in 1 kg water = 1 m (molality). For dilute aqueous solutions, M ≈ m because 1 L water ≈ 1 kg.
Use our Molarity and Molality calculators in the Chemistry category to practice with real values.