Last updated: July 3, 2026
Molality Calculator
Creators
Dharmendra SinghReviewers
Creators
Dharmendra SinghReviewers
Quick Answer
Molality is a concentration unit equal to moles of solute per kilogram of solvent. Calculate it by converting solute mass to moles with molar mass, then dividing by solvent mass in kilograms. To solve backward, multiply molality by solvent kilograms to get solute moles.
Molality equals moles of solute divided by kilograms of solvent, measured in moles per kilogram. It is mass based, so it does not change with temperature the way molarity can.
Key Takeaways
- Molality equals moles of solute divided by kilograms of solvent: b = n / kg solvent.
- Convert solute mass to moles with n = mass ÷ molar mass before dividing.
- Molality uses solvent mass, not final solution volume or total solution mass.
- Given target molality, moles of solute = molality × kilograms of solvent.
- Molality is temperature independent because mass does not expand like volume.
Creators
Dharmendra SinghReviewers
Creators
Dharmendra SinghReviewers
Formula
b = n_solute / m_solvent(kg); n_solute = mass_solute / molar_mass
Where:
- b=Molality(mol/kg)
- n_solute=Amount of solute(mol)
- m_solvent,kg=Mass of solvent(kg)
- m_solute=Mass of solute(g)
- M_w=Molar mass of solute(g/mol)
Worked Examples
1 molal sodium chloride
58.44 g NaCl (molar mass 58.44 g/mol) in 1 kg water.
- 1Convert solute mass to moles: n = 58.44 g ÷ 58.44 g/mol = 1 mol.
- 2Use solvent mass in kilograms: 1 kg.
- 3Divide moles by solvent kilograms: b = 1 mol ÷ 1 kg = 1 mol/kg.
Glucose in half a kilogram of solvent
90 g glucose (180.16 g/mol) dissolved in 0.5 kg solvent.
- 1Convert mass to moles: n = 90 g ÷ 180.16 g/mol ≈ 0.4996 mol.
- 2Solvent mass is 0.5 kg.
- 3Molality is 0.4996 ÷ 0.5 ≈ 0.999 mol/kg.
Target 2 mol/kg solution
Find solute moles for a 2 mol/kg solution using 0.25 kg solvent.
- 1Rearrange the definition: n = b × kg solvent.
- 2Substitute values: n = 2 mol/kg × 0.25 kg.
- 3The required amount of solute is 0.5 mol.
Introduction
Molality, symbol b (often written lowercase m in classrooms), is the amount of solute in moles divided by the mass of solvent in kilograms. Unlike molarity, molality uses solvent mass rather than solution volume, so it is especially useful when temperature changes would alter volume. This calculator applies the IUPAC-compatible definition of amount per solvent mass, converts grams to moles using molar mass, and can also work backward from a target molality. For broader solution composition comparisons, see the mass percent calculator and IUPAC terminology in the Gold Book.
What is molality?
Molality is a concentration measure defined as moles of solute per kilogram of solvent. A 1 mol/kg solution contains 1 mole of dissolved solute for every 1 kg of solvent. The denominator is solvent mass only, not total solution mass, which distinguishes molality from mass percent and from volume-based molarity.
- Unit:
mol/kg.
- Symbol:
b in IUPAC notation; many textbooks use m.
- Denominator:
kilograms of solvent only.
Best for temperature-dependent physical chemistry calculations.
Molality formula explained
The core equation is b = n_solute / kg_solvent. If your solute amount starts as a mass, first convert grams to moles using n = mass ÷ molar mass. Substitution gives b = (mass_solute / molar_mass) ÷ kg_solvent. To solve backward, multiply molality by solvent kilograms to obtain moles of solute.
Use kilograms for solvent mass. If you measured 500 g of solvent, enter 0.5 kg.
How to calculate molality step by step
Begin by identifying the solute and solvent. Weigh the solute, look up or calculate its molar mass, and weigh the solvent separately. Convert solute grams to moles, convert solvent mass to kilograms if needed, then divide moles by kilograms. If you already know the target molality, use n = b × kg solvent to find the required moles before converting to grams.
Record solute mass in grams.
Find solute molar mass in g/mol.
Calculate moles of solute.
Record solvent mass in kilograms.
Divide moles by kilograms of solvent.
Molality vs molarity
Molarity is moles per litre of solution, while molality is moles per kilogram of solvent. Molarity is convenient for volumetric lab preparation and titrations; molality is preferred for colligative properties such as freezing-point depression and boiling-point elevation. For dilution workflows involving volumes, the dilution factor calculator may be more appropriate.
| Measure | Formula | Depends on temperature? |
|---|---|---|
| Molality | mol solute / kg solvent | No, mass based |
| Molarity | mol solute / L solution | Yes, volume based |
| Mass percent | solute mass / solution mass × 100 | No, mass based |
| Mole fraction | moles component / total moles | No, ratio based |
Common molality examples
Molality values depend strongly on solubility and the solvent chosen. A 1 molal aqueous NaCl solution uses 58.44 g NaCl per kilogram of water. A 1 molal glucose solution uses 180.16 g glucose per kilogram of water. Osmolality in clinical chemistry is related but counts osmotically active particles rather than formula units.
For electrolytes such as NaCl, molality of formula units is not the same as particle molality after dissociation.
Accuracy tips and common mistakes
The most common mistake is dividing by total solution mass or by solution volume. Molality requires solvent mass in kilograms. Another error is using the wrong molar mass for hydrates; magnesium sulfate heptahydrate and anhydrous magnesium sulfate have different molar masses. For SI unit conventions, consult the BIPM SI Brochure, and for thermodynamic concentration conventions see the IUPAC Green Book.
Use solvent mass, not solution mass.
Convert grams of solvent to kilograms before calculating.
Use the exact formula mass for hydrates or salts.
Report temperature when comparing with volume-based concentrations.
Quick Reference Card
Molality — Quick Reference
Quick reference • Molality Calculator
b = n_solute / kg solvent = (mass / molar mass) / kg solventValid range: 0 mol/kg to the solubility limit for the solute-solvent pair
Common Values
⚠ Watch Out
- •Do not divide by litres of solution; that is molarity.
- •Do not divide by total solution mass; molality uses solvent mass only.
- •Convert solvent grams to kilograms before calculating.
- •Use the correct molar mass, especially for hydrated salts and solvates.
Pro Tips
- →Use molality for boiling-point elevation and freezing-point depression problems.
- →Keep more digits through the moles step, then round the final molality.
- →When preparing a target molality, weigh solvent first because the denominator is fixed by solvent mass.
- →For electrolytes, distinguish formula-unit molality from ion or osmotic particle molality.
FAQs
What is molality in simple terms?
Molality is the number of moles of solute dissolved per kilogram of solvent. A 1 mol/kg solution has one mole of solute for every kilogram of solvent.
How do I calculate molality from grams?
Divide solute grams by molar mass to get moles, then divide those moles by the solvent mass in kilograms.
Is molality the same as molarity?
No. Molality uses kilograms of solvent; molarity uses litres of final solution. Molality is mass based and does not change with temperature-induced volume expansion.
Why is solvent mass used instead of solution mass?
Molality is defined relative to the solvent because many thermodynamic properties depend on the amount of solute dispersed in a known mass of solvent.
Can this calculator find solute moles from a target molality?
Yes. Enter the target molality and solvent mass. The calculator uses moles of solute = molality × kilograms of solvent.
How do I convert target molality to grams of solute?
First calculate moles using n = b × kg solvent, then multiply by molar mass. Provide molar mass in the calculator to obtain the solute mass output.