# Molality

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 $\small \left (C_{6}H_{6} \right )$.

Solution:

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.

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

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

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