🧪 Molality Calculator
Compute solution molality (mol/kg) from solute amount and solvent mass, complete with optional mass percent insights.
Direct mole entry is ideal for stoichiometry-based preparations.
Mass of solvent only (exclude solute mass).
Provide to estimate mass percent (total solution mass).
Molality is independent of temperature and volume changes, making it ideal for colligative property calculations.
Molality
0.3333 m
Moles of solute per kilogram of solvent.
Solute Amount
0.25 mol
Determined from input mode.
Solvent Mass
0.75 kg
Converted from grams.
Using 0.25 mol of solute and 0.75 kg of solvent yields a molality of 0.3333 mol/kg.
How to Use This Calculator
Select solute input format
Choose between entering moles directly or providing solute mass with molar mass for conversion.
Weigh the solvent separately
Molality depends on the mass of solvent alone. Exclude the solute mass when recording the solvent weight.
Enter the data and compute
Type the values into the form. The calculator instantly reports molality and supporting figures.
Use optional mass percent
Provide total solution mass to estimate a mass-percent concentration alongside molality.
Formula
m = n_solute / m_solvent(kg)
Molality (m) is the ratio of solute moles (n) to kilograms of solvent (m_solvent). For mass inputs, n = mass_solute / molar_mass.
Example
8.00 g NaCl dissolved in 0.250 kg water: n = 8.00 g / 58.44 g/mol = 0.137 mol. Molality = 0.137 / 0.250 = 0.548 m.
Full Description
Molality expresses solution concentration in terms of solute moles per kilogram of solvent. Unlike molarity, it does not change with temperature, making it the preferred measure for freezing point depression, boiling point elevation, and osmotic pressure calculations.
The calculator caters to common laboratory workflows: enter solute moles directly from stoichiometric planning or convert from grams when weighing reagents. The optional mass-percent output helps connect molality to other reporting formats.
Use this tool to verify solution recipes, size colligative property experiments, or convert between concentration units for quality control documentation.
Frequently Asked Questions
Why is solvent mass used instead of solution volume?
Solvent mass stays constant with temperature, whereas volume can expand or contract. Molality remains reliable for thermodynamic calculations.
Can I enter solvent mass in kilograms?
The input expects grams for convenience. The calculator automatically converts grams to kilograms internally.
What if I only know solution mass?
Subtract the solute mass from the total solution mass to obtain the solvent mass before using the calculator.
Does the calculator support mixtures with multiple solutes?
Molality is defined per solute species. Treat each solute separately or sum moles of identical solute contributions.
How accurate are the results?
Accuracy depends on your input measurements. Use calibrated balances and precise molar mass values to minimise uncertainty.