Question

The unit of molality is:
A. Mol/L
B. L/mol
C. Mol/Kg
D. K

Answer 1

For this problem, we should know about the molality and also its formula. By writing the formula we can easily determine the unit of the molality from the given options.

Complete Step-by-Step answer:
- In the given question, we have to choose the correct unit of the molality among the given options.
- As we know that molality is defined as the ratio of the number of moles of solute to the mass of the solvent in kilograms.
- Molality is generally denoted by 'm'.
- The formula of molality is: $\text{Molality = }\dfrac{\text{Number of moles of solute}}{\text{Mass of solvent (Kg)}}$
- Also, the number of moles is the ratio of mass to the molecular mass of the compound i.e. $\text{Number of moles = }\dfrac{\text{mass}}{\text{molecular weight}}$
- So, the expanded form of the formula of molality will be:
$\text{Molality = }\dfrac{\text{Mass }}{\text{Molecular mass }\times \text{ mass of solvent}}$
$\text{Molality = }\dfrac{\text{mol}}{\text{kg}}$
- So, the unit of the molality will be mol/Kg.
- Whereas molarity is the ratio of the number of moles of solute to the volume of the solution in litres.
- Molarity is denoted by 'M' and has unit mol/L.
- Molality and molarity are different from each other because molarity depends on the volume due to which with the change in the temperature the molarity also changes.
- But molality is independent of temperature and does not change with change in the temperature.
Therefore, option C is the correct answer.

Note: Due to the independent nature of molality, it is widely used in many places over molarity such as in colligative properties that are used to determine the solute concentration.