Molality

Molality is one of the important properties of solutions. It is used to express the concentration of a solute in a solution and mostly depends on the mass of the solvent. Molality is also referred to as molal concentration. It is usually denoted by the letter “m”.

The concept of molality was basically framed as a relation to molarity which is described as the molar concentration of a solution. The use of this property was first recorded in the publication authored by G. N. Lewis and M. Randall in the year 1923. The topic was covered in the book titled “Thermodynamics and the Free Energies of Chemical Substances.”  We will learn about the concept in detail below.

Molality Definition and Formula

Molality (m) is defined as the number of moles of solute per kilogram of solvent. The formula for molality is given as,

Molality (m) = moles of solute/kilograms of solvent

Students should remember that molality is used to measure the moles in relation to the mass of the solvent and not the mass of the solution.

There are also instances where the solutions contain more than one solvent. In such cases, we consider or define the molality of the mixed solvent. Alternatively, the units are expressed as mole solute per kilogram mixed solvent.

SI Unit of Molality

The SI unit of molality is moles per kilogram (mol/kg). For example, a solution whose molality is given as 6 mol/kg is stated as 6 molal or 6 m. 

Advantages and Disadvantages of Using Molality

The advantage of using molality as a measure of concentration is that molality, as mentioned above, is solely dependent on the mass of the solute and the solvent. It means that these factors are not affected by changes in temperature or even pressure, as in the case of solutions prepared volumetrically. Secondly, it is quite useful because, in a solution, the molality of one solute is independent of the existence of other solutes.

Coming to the disadvantages, it is not applicable in instances where there is no pure substance in a mixture. For example, mixtures such as water and alcohol or alloys. Here, any one of the substances can be considered as the solvent.

Molality vs Molarity

Molality and Molarity are terms that are often confused, and some students even use them interchangeably. However, these are two different measures to represent the concentration of a chemical solution. They are also used in different instances. We will look at the difference between molality and molarity below.

However, the only case when molality and molarity are similar is when we talk about weak aqueous solutions. Generally, if we take one kilogram of water (solvent), it will occupy a volume of 1 litre at room temperature. As far as the principle is concerned, a small amount of solute will have minimal effect on the volume.

How to Calculate Molality?

Molality is mainly used in expressing the concentrations of solutions related to vapour pressure and temperature changes. Particularly, molality is also used in determining the boiling or melting point and when we are working with colligative properties. As for the calculation, molality can be determined easily by knowing the mass of the solute and solvent in a solution. Moreover, the concentration of a homogeneous solution or molality always remains constant.

Molality Examples

Question: Calculate the molality of a solution where 0.5 grams of toluene (C₇H₈) is dissolved in 225 grams of Benzene (C₆H₆).

Solution:

Calculate the moles of the given solute.

Toluene – Molecular weight

Using the formula:

Moles of toluene

So, the mole of toluene is 0.054 mole.

Now, calculate the kilogram of solvent.

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

Molality (m)

Molality = 0.24 m

Learn more about other expressions, such as Normality for measuring the concentration of solutions, at BYJU’S.

Solutions and Concentration Terms