Last updated: 2026-06-15

Understanding molality

Molality is a concentration measure that expresses the amount of solute relative to the mass of solvent. It is defined as moles of solute per kilogram of solvent, and given the symbol b (or sometimes a lowercase italic m in older texts). Because mass does not change with temperature, molality is the concentration unit of choice whenever temperature is a variable in the problem — most importantly for colligative properties such as boiling-point elevation and freezing-point depression.

The molality formula

$$b = \frac{n_{\text{solute}}}{m_{\text{solvent}}}$$

where $n_{\text{solute}}$ is the amount of solute in moles and $m_{\text{solvent}}$ is the mass of solvent in kilograms. The unit of molality is mol/kg, sometimes written with the informal symbol "m" (molal).

Term	Symbol	Meaning
Molality	$b$	mol of solute per kg of solvent
Moles of solute	$n_{\text{solute}}$	Amount of dissolved substance
Solvent mass	$m_{\text{solvent}}$	Mass of solvent only, in kilograms

Rearranged, the same formula gives the other two quantities:

• Moles of solute = molality × solvent mass (kg)
• Solvent mass (kg) = moles of solute ÷ molality

Worked example

Dissolve 0.5 mol of NaCl in 500 g of water. What is the molality?

$$\begin{aligned} m_{\text{solvent}} &= 500\ \text{g} = 0.5\ \text{kg} \\ b &= \frac{n_{\text{solute}}}{m_{\text{solvent}}} \\ &= \frac{0.5\ \text{mol}}{0.5\ \text{kg}} \\ &= 1.0\ \text{mol/kg} \end{aligned}$$

The solution has a molality of 1.0 mol/kg (1.0 m).

Molality vs. molarity

Both molality and molarity describe how much solute is in a solution, but they use different denominators.

Property	Molality (b)	Molarity (c)
Denominator	kg of solvent	L of solution
Temperature-dependent?	No	Slightly (volume expands)
Needs solvent density?	No	Yes, to convert mass to vol
Used for	Colligative properties	Reaction stoichiometry

For dilute aqueous solutions at room temperature, molality and molarity are numerically close because 1 L of water has a mass of approximately 1 kg. The difference becomes significant at higher concentrations or at temperatures far from 25 °C.

Why molality matters for colligative properties

Colligative properties — boiling-point elevation, freezing-point depression, osmotic pressure, and vapour-pressure lowering — depend on the ratio of solute particles to solvent particles. The relevant equations are:

$$\Delta T_b = K_b \cdot b \qquad \Delta T_f = K_f \cdot b$$

where $K_b$ and $K_f$ are solvent-specific constants (for water, $K_b = 0.512\ \text{°C·kg/mol}$ and $K_f = 1.86\ \text{°C·kg/mol}$) and $b$ is the molality. Molarity cannot substitute here because when a solution boils or freezes, its temperature and volume change — which would alter a molarity-based result during the very process being calculated.

Converting from grams of solute to moles

Most laboratory preparations start from a mass of solute rather than moles directly. The conversion requires the molar mass $M$:

$$n_{\text{solute}} = \frac{m_{\text{solute}}}{M}$$

For example, 18 g of glucose (molar mass 180 g/mol) gives $n = 18 \div 180 = 0.10\ \text{mol}$. Dissolved in half a kilogram of water, the molality is $0.10 \div 0.5 = 0.20\ \text{mol/kg}$.

Frequently Asked Questions (FAQ)

What is the molality formula?

Molality (symbol b) is the amount of solute in moles divided by the mass of solvent in kilograms: b = n ÷ m(solvent). Unlike molarity, the denominator is the solvent mass alone — not the total solution volume. For example, dissolving 0.5 mol of NaCl in 1 kg of water gives a molality of 0.5 mol/kg, written 0.5 m.

What is the difference between molality and molarity?

Molarity (c or M) is moles of solute per litre of solution. Molality (b or m) is moles of solute per kilogram of solvent. The practical difference is temperature: solution volume changes as temperature rises, so molarity shifts slightly with temperature. Solvent mass does not change, so molality is constant across temperatures. When temperature matters — such as in boiling-point and freezing-point calculations — molality is the preferred concentration unit.

Why is molality used for colligative properties?

Colligative properties — boiling-point elevation, freezing-point depression, osmotic pressure, and vapour-pressure lowering — depend on the ratio of solute particles to solvent particles. Because solvent mass is proportional to the number of solvent molecules (for a pure solvent), molality directly captures that ratio and stays constant regardless of temperature. Using molarity would introduce a small error because the solution volume, and therefore the denominator, shifts as the temperature changes during boiling or freezing.

How do I calculate molality step by step?

Step 1 — find moles of solute: divide the solute mass by its molar mass. For 18 g of glucose (molar mass 180 g/mol), n = 18 ÷ 180 = 0.1 mol. Step 2 — convert solvent mass to kilograms: if you have 500 g of water, that is 0.5 kg. Step 3 — divide: b = 0.1 mol ÷ 0.5 kg = 0.2 mol/kg. The result is sometimes written 0.2 m (lowercase italic m, not to be confused with meters).