Molality
Molality (m) is a measure of solution concentration expressed as the number of moles of solute per kilogram of solvent (not solution). Unlike molarity, molality is independent of temperature and pressure because it is based on mass rather than volume, making it ideal for studying colligative properties such as boiling point elevation and freezing point depression. It is the preferred concentration unit in thermodynamic calculations involving solutions.
Key Formula
m = moles of solute / mass of solvent (in kg)
LaTeX: m = \dfrac{n_{\text{solute}}}{m_{\text{solvent (kg)}}}
| Symbol | Meaning | Unit |
|---|---|---|
| m | Molality | mol/kg (molal, denoted m) |
| n_solute | Moles of solute | mol |
| m_solvent | Mass of solvent | kg |
Worked Example
Problem
Calculate the molality of a solution made by dissolving 10.0 g of glucose (C₆H₁₂O₆, M = 180.16 g/mol) in 250.0 g of water.
Solution
Step 1: Convert mass of glucose to moles: n = 10.0 g / 180.16 g/mol = 0.05550 mol. Step 2: Convert mass of water to kg: 250.0 g / 1000 = 0.2500 kg. Step 3: Calculate molality: m = 0.05550 mol / 0.2500 kg = 0.2220 mol/kg.
Answer
Molality = 0.222 m (mol/kg)
Molality vs Molarity: Key Differences
| Property | Molality (m) | Molarity (M) |
|---|---|---|
| Definition | mol solute / kg solvent | mol solute / L solution |
| Temperature dependence | Independent | Dependent (volume changes) |
| Units | mol/kg (molal) | mol/L (molar) |
| Best used for | Colligative properties | Most solution stoichiometry |
| Pressure dependence | Independent | Slight dependence |
| Solvent or solution? | Uses mass of solvent | Uses volume of solution |
Interactive Tools
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Related Terms
Molarity
Molarity (M) is the most common measure of solution concentration, defined as the number of moles of solute dissolved per litre of solution. It is temperature-dependent because liquid volumes change with temperature. Molarity is widely used in titrations, reaction stoichiometry involving solutions, and the preparation of standard laboratory solutions.
Mole Fraction
Mole fraction (χ) is a dimensionless concentration unit expressing the ratio of the moles of one component to the total moles of all components in a mixture. The sum of all mole fractions in a mixture always equals exactly 1. Mole fraction is used in Raoult's law, Dalton's law of partial pressures, and chemical thermodynamics because it is independent of temperature and pressure.
Henry's Law
Henry's Law states that the amount of a gas dissolved in a liquid at constant temperature is directly proportional to the partial pressure of that gas above the liquid. It applies to dilute solutions of gases that do not react chemically with the solvent. Henry's Law explains why carbonated beverages fizz when opened (pressure release), and is critical in understanding oxygen transport in blood, gas exchange in lungs, and environmental fate of volatile compounds.
Derived from Latin "moles" (mass) with the suffix "-ality" denoting a property or state. The term was introduced in the early 20th century to distinguish mass-based concentration from the volume-based molarity.