Molality is defined as:

• A. (moles of solute / L of solution)
• B. (moles of solvent / kg of solvent)
• C. (grams of solute / kg of solvent)
• D. (moles of solute / kg of solvent)

Answer: (D)

Molality measures how many moles of solute are present per kilogram of solvent. It is defined as m = n_solute / m_solvent, with n_solute in moles and m_solvent in kilograms. This choice is best because mass stays essentially constant with temperature, while volume can change, so molality remains the same as conditions change. That stability is why molality is particularly useful for colligative properties like boiling-point elevation and freezing-point depression. Other concentration measures use liters of solution or grams of solute, which depend on temperature or total solution mass, so they don’t have this temperature-insensitive property. For example, dissolving 2 moles of solute in 1 kilogram of solvent gives a molality of 2 mol/kg.