Chemistry overview
What is molality?
Molality measures the number of moles of dissolved solute in each kilogram of solvent.
Unlike concentration measures based on solution volume, molality uses the mass of the solvent alone.
Formula
Molality formula
Molalitym = n ÷ kg of solvent
- m is molality in moles per kilogram.
- n is the amount of dissolved solute in moles.
- The denominator is solvent mass in kilograms, not total solution mass.
Calculation method
How to calculate molality
- Determine the amount of solute in moles.
- Identify the mass of solvent separately from the solute.
- Convert solvent mass from grams to kilograms when necessary.
- Divide solute moles by kilograms of solvent.
Worked example
Molality calculation example
A solution contains 0.25 mol of solute dissolved in 500 g of solvent.
- Convert solvent mass: 500 g ÷ 1000 = 0.5 kg
- Apply the formula: 0.25 ÷ 0.5 = 0.5
- Report the result as 0.5 mol/kg
The solution therefore has a molality of 0.5 mol/kg.
Rearranged equations
Find solute moles or solvent mass
Solute molesn = m × kg of solvent
Solvent masskg of solvent = n ÷ m
The calculator automatically rearranges the equation according to the selected unknown variable.
Concentration comparison
Molality versus molarity
Molality divides solute moles by kilograms of solvent. Molarity divides solute moles by liters of total solution.
- Molality unit: mol/kg of solvent.
- Molarity unit: mol/L of solution.
- Molality depends on mass.
- Molarity depends on volume.
Use the Molarity Calculator when the problem provides total solution volume rather than solvent mass.
Temperature behavior
Why molality is less temperature-dependent
Mass normally remains constant when temperature changes, while liquid volume can expand or contract.
Molality is therefore commonly used in calculations where temperature changes could make volume-based concentration less stable.
Common errors
Common molality mistakes
- Using total solution mass instead of solvent mass.
- Entering grams without converting them to kilograms.
- Using solute mass in grams instead of solute amount in moles.
- Confusing molality with molarity.
- Rounding intermediate values too early.
Model assumptions
Assumptions and limitations
- Solute amount must be expressed in moles.
- Solvent mass must be greater than zero.
- Solvent mass must exclude the mass of the dissolved solute.
- The calculator reports composition but does not model activity coefficients or non-ideal solution behavior.
- Result accuracy depends on the accuracy of the entered amounts.
Supporting tools
Prepare the required inputs
Convert solute mass to moles with the Mass to Moles Calculator, or determine molar mass using the Molecular Weight Calculator.
For mixture composition based on component and total moles, use the Mole Fraction Calculator.
Questions and answers
Molality calculator FAQ
What is molality?
Molality is the number of moles of solute divided by the mass of solvent in kilograms. Its standard unit is moles per kilogram, written as mol/kg.
What formula does the molality calculator use?
The calculator uses m = n divided by kilograms of solvent, where n is the amount of solute in moles and m is molality in mol/kg.
Should solvent mass be entered in grams or kilograms?
Either unit can be used. The calculator automatically converts grams to kilograms before applying the molality formula.
Is molality the same as molarity?
No. Molality uses kilograms of solvent, while molarity uses liters of total solution. They describe concentration using different denominators.
Does temperature affect molality?
Molality is based on mass rather than volume, so it is generally less sensitive to temperature-driven expansion or contraction than molarity.