Question

Why molality is preferred over molarity in expressing the concentration of solution?

Answer 1

We have to remember that the molality is defined as the number of moles of solute per kilogram of solute. It is a property of solutions. Molarity is defined as the number of moles of solute per litre of solution. Volume changes with temperature but mass doesn’t.

Complete step by step answer:
Let’s start with discussing the concept of molality and molarity for better understanding of the question. Molality is defined as the number of moles of solute per kilogram of solute. It is a property of solutions. Molarity is defined as the number of moles of solute per litre of solution.
In the question we are asked about the reason why molality is preferred over molarity for expressing concentrations. Well molarity is the function of volume and due to which it is affected by the temperature of the solution whereas the molality is the function of mass and as we know the mass of the substance is not affected by the temperature of the solution. So, when there is a change in temperature then the molality always remains constant but the molarity changes.
Hence we prefer molality over molarity in expressing the concentration of solutions.

Note: We must remember that the molality is preferred over molarity but still while making solution we made the solution in molarity and the reason behind this is we know that at room temperature there will not be any immediate change in temperature, also it is relatively easy to measure the volume of liquid rather than weighing it.