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

Sometimes 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

Talking about the advantages of using molality as a measure of concentration is that molality, as mentioned above, is solely dependent on the mass of solute and the solvent. What it means is 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, anyone of the substance can be considered as the solvent.

Molality vs Molarity

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

Molality Molarity
It is the number of moles of solute per kilogram of solvent.  It is the number of moles of solute per kilogram of solvent. 
Change in temperature will not affect this unit of concentration. The unit will change with change in temperature.
Its unit is mol/kg. Its unit is mol/litre.
Denoted by the letter (m). Denoted by the letter (M).

Meanwhile, 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 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 \((C_{7}H_{8})\) is dissolved in 225 grams of Benzene \((C_{6}H_{6})\).

Solution

Calculate the moles of given solute.

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

Using the formula:

Moles of Toluene = \(\frac{Mass\; in \;grams}{Molecular\; weight}\) = 0.054 mole.

So, the mole of toluene is 0.054 mole.

Now calculate the kilogram of solvent.

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

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

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

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

Molality = 0.24 m

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

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