Molality is used when dealing with colligative properties because it uses the solvent's mass (in kilograms) rather than the solution's volume.

Molality is used when dealing with colligative properties because it uses the solvent's mass (in kilograms) rather than the solution's volume.

• A. Moles of solute per kilogram of solvent
• B. Moles of solute per liter of solution
• C. Mass percent of solute
• D. Moles of solvent per mole of solute

Answer: (A)

Coligative properties depend on the number of solute particles in a given amount of solvent, not on the identity of the particles. Volume, which affects concentrations like molarity, changes with temperature, making such measures less reliable for these properties. Using molality—moles of solute per kilogram of solvent—keeps the concentration based on solvent mass, which stays constant with temperature changes. This makes molality a stable pará meter for predicting how boiling point, freezing point, and osmotic pressure shift. The other options describe concentration by volume (molarity), by mass fraction (mass percent), or invert the ratio (solvent per solute), none of which aligns with how colligative effects are quantified.

Coligative properties depend on the number of solute particles in a given amount of solvent, not on the identity of the particles. Volume, which affects concentrations like molarity, changes with temperature, making such measures less reliable for these properties. Using molality—moles of solute per kilogram of solvent—keeps the concentration based on solvent mass, which stays constant with temperature changes. This makes molality a stable pará meter for predicting how boiling point, freezing point, and osmotic pressure shift. The other options describe concentration by volume (molarity), by mass fraction (mass percent), or invert the ratio (solvent per solute), none of which aligns with how colligative effects are quantified.