Molarity vs. Molality: The Core Distinction
The fundamental difference between molarity and molality lies in what the amount of solute is compared to. Molarity (M) is the number of moles of solute per liter of the total solution. In contrast, molality (m) is the number of moles of solute per kilogram of the solvent.
Section 2: Defining the Denominators
For molarity, the denominator is the volume of the entire solution (solute + solvent). This value can change with temperature and pressure, as the volume of a liquid can expand or contract. For molality, the denominator is the mass of just the solvent, which does not change with temperature or pressure.
Section 3: A Practical Example
Imagine you dissolve 1 mole of sugar in water. To make a 1 M (molar) solution, you would add enough water to make the total volume exactly 1 liter. To make a 1 m (molal) solution, you would dissolve the sugar in exactly 1 kilogram of water, regardless of the final total volume.
Section 4: Why the Difference Matters
Because molality is independent of temperature changes, it is preferred in physical chemistry for calculations involving colligative properties like freezing point depression and boiling point elevation. Molarity is more commonly used in general lab preparations where temperature is constant and measuring volume is more convenient than mass.