Definition
Molality (m) is defined as the number of moles of solute per kilogram (kg) of solvent. Unlike molarity, molality is independent of temperature because it is based on mass rather than volume.
Formula:
Unit:
The unit of molality is moles per kilogram (mol/kg), often represented as m (e.g., 2 m NaCl).
Calculation Example:
Calculate the molality of a solution prepared by dissolving 10 g of NaCl in 200 g of water.
Solution:
Answer: The molality of the NaCl solution is 0.855 m.
Advantages of Molality
- Temperature-Independent: Since molality is based on mass, it remains constant even when temperature changes (unlike molarity).
- Useful in Colligative Properties: Molality is used in boiling point elevation, freezing point depression, and osmotic pressure calculations.
- Ideal for Highly Concentrated Solutions: In cases where volume contraction occurs, molality provides more accurate measurements than molarity.
Factor affecting Molarity
Molarity (M) is the concentration of a solution expressed as the number of moles of solute per liter of solution. Several factors affect molarity, including:
1. Amount of Solute (n):
- Increasing the number of moles of solute increases molarity.
- Decreasing the solute amount lowers molarity.
2. Volume of Solution (V):
- Increasing the total volume of the solution decreases molarity.
- Decreasing the volume increases molarity.
3. Temperature:
- Temperature changes can expand or contract the solution volume.
- Higher temperatures generally increase volume, lowering molarity.
- Lower temperatures decrease volume, increasing molarity.
4. Nature of the Solvent:
- Some solvents interact with solutes differently, affecting solubility and final solution volume.
5. Dissociation of Solutes:
- If the solute dissociates (e.g., NaCl → Na⁺ + Cl⁻), the number of particles in solution increases, affecting molarity in ionic solutions.
Conclusion
Molality is a crucial concentration unit in pharmaceutical and chemical analysis, especially when working with temperature-sensitive solutions. Its independence from volume changes makes it ideal for studying colligative properties and precise formulations. Understanding molality helps ensure accurate dosing, solution preparation, and quality control in the pharmaceutical analysis industry.
