The difference between molarity and molality lies mainly in how concentration is measured:
- Molarity (M):
- Defined as the number of moles of solute per liter of solution.
- Formula: M = (moles of solute) / (volume of solution in liters).
- Depends on volume, which can change with temperature and pressure (since liquids expand or contract).
- Example: A 1 M NaCl solution means 1 mole of NaCl is dissolved in enough water to make 1 liter of solution.
- Molality (m):
- Defined as the number of moles of solute per kilogram of solvent.
- Formula: m = (moles of solute) / (mass of solvent in kilograms).
- Depends only on mass, which does not change with temperature or pressure.
- Example: A 1 m NaCl solution means 1 mole of NaCl is dissolved in 1 kilogram of water.
Key Difference:
- Molarity = moles per liter of solution (volume-based, temperature dependent)
- Molality = moles per kilogram of solvent (mass-based, temperature independent)