Question

Explain the difference between molarity and molality and their temperature dependence.

Hint:

Molarity = Volume based (temperature dependent); Molality = Mass based (temperature independent).

Answer 1

Concept: Molarity and molality are concentration terms, but they differ in how they depend on volume and mass, which affects temperature dependence.
Answer:
Molarity (M): \[ M = \frac{\text{moles of solute}}{\text{volume of solution in liters}} \] Molality (m): \[ m = \frac{\text{moles of solute}}{\text{mass of solvent in kg}} \] Differences:

• Basis: Molarity depends on volume; Molality depends on mass.
• Temperature Effect: Molarity changes with temperature (volume expands or contracts), whereas molality remains constant (mass unaffected).
• Usage: Molality is preferred in colligative properties due to temperature independence.

Temperature Dependence:
As temperature increases, the volume of solution increases, causing molarity to decrease. However, molality remains unchanged because mass does not vary with temperature.