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Molality is moles of solute divided by the mass of the solvent in kilograms: b = n(solute) / m(solvent), in mol/kg. The denominator is the solvent alone, not the finished solution, and that single detail is what separates the molal formula from the molarity formula. NIST’s current guidance also asks authors to write “molality” and “mol/kg” rather than the older shorthand “molal” and “m”.
Short answer
The molality equation is bB = nB / msolvent, where nB is the amount of solute B in moles and msolvent is the mass of the solvent in kilograms. The result is expressed in mol/kg. NIST’s Guide to the SI defines molality of solute B as the amount of substance of solute B in a solution divided by the mass of the solvent.
Definition and scope: what is molal, and what is molality?
“Molal” is the traditional adjective for a solution whose concentration is stated per kilogram of solvent, as in “a 2 molal solution”. The quantity itself is molality. In the SI conventions published by NIST, the quantity is called the molality of solute B, its symbol is bB (also mB), and its SI unit is the mole per kilogram (mol/kg), or a decimal multiple or submultiple of that unit.
NIST states plainly that “the term molal and the symbol m should no longer be used because they are obsolete.” The same chapter says the term molarity and the symbol M are obsolete too, and recommends “amount-of-substance concentration of B” instead. Older textbooks and many lab labels still use “molal” and “molar”, so you need to read both styles, but new documents should use the current terms.
Molal units, molality units and the unit of molality
All the common phrasings of this question (molal units, molality unit, unit for molality, unit of molality) have one answer: mol/kg. Decimal multiples such as mmol/kg are acceptable where the numbers are small. The table compares the two concentration quantities that are most often confused. The entries follow NIST’s SI guidance.
| Quantity (NIST name) | Symbol | Unit | Obsolete name to avoid |
|---|---|---|---|
| Molality of solute B | bB (also mB) | mol/kg | molal, symbol m |
| Amount-of-substance concentration of B | cB | mol/dm3, kmol/m3 or mol/L (1 mol/dm3 = 1 kmol/m3 = 1 mol/L) | molarity, symbol M |
How is molality calculated?
Divide the moles of solute by the kilograms of solvent. If you start from a mass of solute, first convert it to moles using the solute’s molar mass, then apply the formula. The two worked examples below use illustrative numbers, not measured data.
Worked example 1: solvent mass in the denominator
Suppose 0.50 mol of a solute is dissolved in 250 g of water. Convert the solvent to kilograms: 250 g = 0.250 kg. Then b = 0.50 mol / 0.250 kg = 2.0 mol/kg. Written the current way, this is a molality of 2.0 mol/kg, not “2.0 m”.
Worked example 2: the common slip of using the solution mass
Now suppose the 0.50 mol of solute has a mass of 50 g. The solution mass is 250 g + 50 g = 300 g = 0.300 kg. Dividing by that number gives 0.50 / 0.300 = 1.67 mol/kg, which is about 17% too low. The definition uses the mass of the solvent only, so the correct value remains 2.0 mol/kg.
Converting units
Because the unit is built from the mole and the kilogram, multiples follow ordinary SI prefix arithmetic: 2.0 mol/kg is 2000 mmol/kg.
Molal vs molar: how the two quantities differ
By the NIST definitions, molality is amount of solute per mass of solvent (mol/kg), while amount-of-substance concentration, the quantity formerly called molarity, is reported in units of amount per volume (mol/L). So the first is a ratio of a mole count to a mass, and the second a ratio of a mole count to a volume. For the volume-based quantity and its everyday calculations, see our guide to molarity and solution calculations for the lab, the comparison moles vs molarity, and the walkthrough converting moles to molarity.
Where it matters: a reporting checklist for methods and labels
NIST’s SI style checklist gives a concrete wording standard that you can apply to methods sections, protocols and bottle labels. Use it as a checklist:
- Name the quantity “molality of solute B” and give the symbol bB or mB, with the unit mol/kg (or a related SI unit).
- Do not use the terms normality, molarity or molal, or the symbols N, M and m for them. NIST lists these as obsolete.
- For a volume-based concentration, write amount-of-substance concentration with mol/L (or mol/dm3) as the unit.
- Name the solute and the solvent next to the number, since the quantity is defined per mass of solvent.
For making working solutions in practice, including buffers, see the buffer and solution preparation guide. For a concentration expressed as a mass fraction, see mass percent solution.
Common misunderstandings
- Using the solution mass. The denominator is solvent mass. Worked example 2 shows how far off the result gets.
- Using grams instead of kilograms. The unit is mol/kg, so convert grams of solvent to kilograms first.
- Treating “molal” and “molar” as the same thing. They name different quantities with different units (mol/kg and mol/L).
- Writing “2 m” or “2 M” in new documents. NIST marks both the term and the symbol as obsolete in each case.
FAQ
What is the molal formula?
b = n(solute) / m(solvent), with the solute amount in moles and the solvent mass in kilograms, giving mol/kg.
What is the unit of molality?
The mole per kilogram (mol/kg), or a decimal multiple or submultiple such as mmol/kg.
What is molal?
“Molal” is the older adjective for a concentration expressed per kilogram of solvent. NIST calls the term and the symbol m obsolete and recommends “molality” with mol/kg.
What is the difference between molal and molar?
Molal refers to moles of solute per kilogram of solvent. Molar refers to moles per litre, a quantity NIST now names amount-of-substance concentration.
How is molality calculated?
Convert the solvent to kilograms, convert the solute to moles, and divide moles by kilograms, as in worked example 1.
Sources
- NIST, Guide for the Use of the International System of Units (SI), SP 811, Chapter 8: sections 8.6.5 and 8.6.8 on molarity and molality. Checked 2026-10-29.
- NIST, SI unit rules and style conventions checklist. Checked 2026-10-29.
