Molality is defined as

Molality is defined as

• A. Moles per liter of solution
• B. Moles per kg of solvent
• C. Grams per liter
• D. Moles per kilogram of solution

Answer: (B)

Molality measures how many moles of solute are present for every kilogram of solvent. The reason for defining it with solvent mass is that mass stays constant with temperature, while volume can change with heat or cooling. This makes molality a temperature-independent measure, which is especially useful for colligative properties like boiling point elevation and freezing point depression that depend on the amount of dissolved particles relative to the solvent. The other descriptions mix volume or include the total solution rather than just the solvent: moles per liter of solution is molarity, grams per liter isn’t a standard concentration, and moles per kilogram of solution would involve both solute and solvent rather than isolating the solvent mass.

Molality measures how many moles of solute are present for every kilogram of solvent. The reason for defining it with solvent mass is that mass stays constant with temperature, while volume can change with heat or cooling. This makes molality a temperature-independent measure, which is especially useful for colligative properties like boiling point elevation and freezing point depression that depend on the amount of dissolved particles relative to the solvent. The other descriptions mix volume or include the total solution rather than just the solvent: moles per liter of solution is molarity, grams per liter isn’t a standard concentration, and moles per kilogram of solution would involve both solute and solvent rather than isolating the solvent mass.