molality
Summary
Molality (m) is moles of solute per kilogram of solvent. Because it uses mass rather than volume, it does not change with temperature, so colligative property equations such as ΔTf = i Kf m use it instead of molarity.
Detail
Meaning: Molality is a concentration unit equal to moles of solute per kilogram of solvent. Because it uses mass instead of volume, it does not change with temperature, while molarity shifts slightly as the solution expands or contracts. That is why colligative property equations use it. In dilute aqueous solutions near room temperature, molality and molarity are nearly equal, since 1 L of water is about 1 kg.
Equation: m = nsolute / msolvent, where m is molality (mol/kg), nsolute is moles of solute (mol), and msolvent is mass of solvent (kg). In colligative problems: ΔTf = i Kf m, with ΔTf the freezing point depression (K), i the van't Hoff factor (unitless), and Kf the solvent's constant (K·kg/mol).
On the MCAT: A passage gives grams of solute and an amount of solvent, then asks for a freezing point depression or boiling point elevation. Convert grams to moles and solvent to kilograms, then multiply by i. You may also rank solutions by i × m to find the lowest freezing point.
Common confusion: Students divide by liters of solution or by the total mass of solution. The denominator is only the solvent's mass in kg. Lowercase m is molality; uppercase M is molarity.
Sources
- Chemistry: The Central Science
- Chemistry: A Molecular Approach
- General Chemistry: Principles and Modern Applications
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.