Difference Between Molarity and Molality

Difference Between Molarity and Molality

In chemistry, the concentration of any solution can be expressed in various ways. The most commonly used concentration units are Molarity (M) and Molality (m). Both depend on the moles of the solute, but they are based on different criteria.

1. Molarity (M)Molarity is defined as:
   The number of moles of solute present in 1 liter or 1000 ml of solution.Formula:
   M=Number of moles of solute/Volume of solution in liters​

   Molarity is temperature-dependent because the volume of the solution changes with temperature.

   Unit of Molarity: mol/L

   Example:
   2 M NaOH means that there are 2 moles of NaOH dissolved in 1 liter of solution.

2. Molality (m)Molality is defined as:
   The number of moles of solute present in 1 kilogram or 1000 grams of solvent.Formula:
   m=Number of moles of solute/Mass of solvent in kilograms​

   Molality is temperature-independent because the mass of the solvent remains constant.

   Unit of Molality: mol/kg or m.

   Example:
   2 m NaCl means that there are 2 moles of NaCl dissolved in 1 kilogram of water.

3. Key Differences (Summary Table)

Feature	Molarity (M)	Molality (m)
Basis	Volume of solution	Mass of solvent
Temperature Dependence	Yes	No
Unit of Concentration	mol/L	mol/kg
Usage	Common laboratory use	Calculation of colligative properties

Conclusion:

Molarity is more convenient for general laboratory and practical applications, while molality is more suitable for theoretical and research work due to its independence from temperature.