Molality is also known as molal concentration. It is a measure of solute concentration in a solution. The solution is composed of two components; solute and solvent. There are many different ways to express the concentration of the solutions like molarity, molality, normality, formality, volume percentage, weight percentage and part per million.

Molality Definition

Molality is defined as the “total moles of a solute contained in a kilogram of a solvent.” The term needs to calculate the mass of the solvent and moles of solute.

Molality Formula

\( Molality \left ( M \right ) = \frac {Number \ of \ moles \ of \ solute}{Mass \ of \ solvent \ in \ kgs}\)

\( Molality \left ( M \right ) = \frac{g \times 1000}{W \times m}\)

Difference Between Molarity and Molality

Molality Molarity
Mass of the solvent Volume of the whole solution
Unit sign expressed as \(\small \left (m \right )\) Unit sign expressed as \(\small \left ( M \right )\)
It has units of \(\small moles / kg\) It has units of \(\small moles / liter \)

Molality Examples

Question: Calculate the molality of a solution where 0.5 grams of toluene \(\small \left (C_{7}H_{8} \right )\) is dissolved in 225 grams of Benzene \(\small \left (C_{6}H_{6} \right )\).


Calculate the moles of given solute.

Toluene: Molecular weight = \(\small C_{7}H_{8} = 7 \times 12 \times + 1 \times 8 = 92 \ moles / gram\)

Using the formula:

\(\small Moles \ of \ Toluene = \frac{Mass\ in\ grams}{Molecular\ weight} = \frac{5.0\ grams}{92\ grams/mole} = 0.054 \ mole.\)

So, the mole of toluene is \(\small 0.054 \ mole.\)

Now calculate the kilogram of solvent.

\(\small \frac{225\ grams\ of Benzene}{1000} = 0.225 \ kilogram\)

As the final step, calculate the molality using the formula.

\(\small Molality \left (m \right ) = \frac{Moles\ of\ Toluene}{Mass\ of\ Benzene\ in\ grams}\)

\(\small Molality \left (m \right ) = \frac{0.054 \ moles}{0.225 \ kg}\)

\(\small Molality \left (m \right ) = 0.24 \ m\)