Concentration Term: Molality
Molality is one of the ways of expressing the concentration of the solution.
Molality (m) is defined as the number of moles of solute divided by the weight of solvent in kg.
We can write the expression for molality as shown above.
Molality, also known as molal concentration is denoted by 'm'.
The concentration term, molality, does not depend on temperature.
Since molality is the ratio of moles to mass, and mass does not change with temperature;
the molality of a solution will remain the same at all temperatures.
Let us understand the concept of molality in detail with the help of an example.
Let us say a solution has
$2.5g$
of ethanoic acid in
$75g$
of benzene.
To find the molality of the solution, we first need to find out the number of moles of ethanoic acid.
The number of moles of ethanoic acid can be found by dividing the given mass by its molar mass.
The Molar mass of ethanoic acid is equal to:
$C+3H+C+O+O+H$
=
$12+3+12+16+16+1=60gmol_{âˆ’1}$
On replacing the values in the expression, we get the number of moles of ethanoic acid to be 0.0417
Finally, we replace the values of number of moles of ethanoic acid and mass of benzene in the molality expression.
Now let us look at the relationship between molality and other concentration terms
If we know the mass of solution, molar mass of the solute and density of the solution, we can easily relate molality and molarity by the expression:
Revision
Molality is a way to express the concentration of a solution.
Molality is the ratio of number of moles of a solute to the weight of solvent in kg.
Molality is independent of temperature.
The end