Molality (m)
Moles of solute dissolved per kilogram of solvent; temperature independent.
What is Molality (m)?
The number of moles of solute dissolved per kilogram (kg) of solvent.
Key formula / rule: Molality
Key points
- Define molality and state its unit.
- Calculate molality given the mass of solute, molar mass of solute, and mass of solvent.
- Convert between molality and other concentration units (e.g., molarity, mass percentage) given density.
- Explain why molality is temperature independent.
Common exam trap
Using mass of solution instead of mass of solvent.
Definitions
- Term
Molality (m)
- Meaning
The number of moles of solute dissolved per kilogram (kg) of solvent.
- Term
Solute
- Meaning
The substance that is dissolved in a solvent to form a solution, usually present in a smaller amount.
- Term
Solvent
- Meaning
The substance in which the solute is dissolved, usually present in a larger amount.
- Term
Solution
- Meaning
A homogeneous mixture composed of two or more substances.
Learning objectives
Define molality and state its unit.
Calculate molality given the mass of solute, molar mass of solute, and mass of solvent.
Convert between molality and other concentration units (e.g., molarity, mass percentage) given density.
Explain why molality is temperature independent.
Differentiate between molality and molarity.
Formulae
- Name
Molality
- Note
Where nsolute is moles of solute and Wsolventkg is the mass of solvent in kilograms.
- Expression
m = (nsolute) / (Wsolventkg)
- Name
Moles of Solute
- Note
To calculate moles of solute from its given mass and molar mass.
- Expression
nsolute = (Masssolute) / (Molarmass_solute)
Prerequisites
Basic understanding of moles and molar mass.
Ability to convert grams to kilograms.
Knowledge of basic solution terminology (solute, solvent, solution).
Stoichiometric calculations.
Common mistakes
Using mass of solution instead of mass of solvent.
Not converting mass of solvent to kilograms.
Confusing molality (m) with molarity (M).
Incorrectly calculating moles of solute (e.g., using molar mass of solvent instead of solute).
Forgetting that molality is temperature independent, while molarity is temperature dependent.
Keywords
Molality
concentration
moles
solvent
solute
temperature independent
colligative properties
mol/kg
Practice preview
What is the molality of a solution containing 30 g of methyl alcohol (CH3OH) in 200 g of water? (Molar mass of CH3OH = 32 g/mol)…
medium
A solution is prepared by dissolving 5.85 g of NaCl in 200 g of water. If the density of the solution is 1.05 g/mL, calculate the molality of the solution. (Molar mass of NaCl = 58.5 g/mol)…
hard
Consider a solution where 10 moles of solute are dissolved in 5 kg of solvent. If the molar mass of the solute is 50 g/mol, what is the molality of the solution?…
easy