[Chemistry Class Notes] on Molality Formula Pdf for Exam

Molality can be used for a measure of concentrations only when there is only one pure, clear solvent in the solution, but in certain cases, when the solution has more than one solvent in it, the problem arises. For example, in aqueous ethanol, when two solvents are present, either one could be treated as a solvent and the calculation, in that case, gets difficult. Not only this, in solid solutions like alloy, molality cannot be taken into calculation, and in that case, mole fraction is preferred over molality.

Molality is defined as the number of moles of solute present in 1000 gm of the solvent. 

For example, 1 molal NaOH solution means a solution with 1 mole of NaOH in 1 Kg water.

Mathematically,

[text{molality(m)} = frac{text{number of moles of Solute(n)}}{text{weight of the solvent in Kg}}].

Here, molality is expressed as m and, mathematically, it is equal to moles of solute present per kilogram of the solution.

It is a basic SI unit of the amount of the substance which can be obtained by dividing the mass of the sample by the molar mass of the compound. 

Mathematically, 

[text{Number of moles} = frac{text{Mass of the given sample}}{text{Molar mass of the element or compound}}].

Molality does not depend on temperature. This is because mass is independent of temperature. 

One key thing to note here is that we are using the weight of the ‘Solvent’ and not ‘Solution’ to calculate molality, which can be an easy mistake to commit while solving the problems.