Question

In which mode of expression, the concentration of a solution remains independent of temperature?

• A. Molarity
• B. Normality
• C. Formality
• D. Molality

Answer 1

The correct option is D

Molality

Explanation for correct option:

Option (d): Molality:

• Molality is a measure of the number of moles of solute in a solution corresponding to 1 kg or 1000 g of solvent.

$\mathrm{molality}\left(\mathrm{m}\right)=\frac{\mathrm{moles}\mathrm{of}\mathrm{solute}}{\mathrm{mass}\mathrm{of}\mathrm{solvent}\mathrm{in}\mathrm{kg}}$

• Molality is determined by the mass of the solute and the solvent.
• The temperature has no effect on mass.
• Hence, morality is independent of temperature.

Explanation for incorrect options:

Option (a): Molarity:

• Molarity is defined as the moles of a solute per litres of a solution.

$\mathrm{molarity}\left(\mathrm{M}\right)=\frac{\mathrm{number}\mathrm{of}\mathrm{moles}\mathrm{of}\mathrm{solute}}{\mathrm{volume}\mathrm{of}\mathrm{solution}}$

• Temperature is directly proportional to Volume, so if temperature changes, it will affect the volume and thus molarity.
• Hence, molarity is dependent on temperature.

Option (b) Normality:

• Normality is related to molarity as:

$\mathrm{Normality}=\mathrm{molarity}\times {\mathrm{n}}_{\mathrm{factor}}$

• So, if molarity varies as temperature changes, normality will also change.

Option (c): Formality:

• The formality of a solution is defined as the number of formula mass of any solute dissolved in 1 litre of solution.

$\mathrm{Formality}\left(\mathrm{F}\right)=\frac{\mathrm{number}\mathrm{of}\mathrm{formula}\mathrm{weight}\mathrm{of}\mathrm{solute}}{\mathrm{volume}\mathrm{of}\mathrm{solution}}$

• Temperature is directly proportional to Volume, so if temperature changes, it will affect the volume and thus formality.
• Hence, formality is also dependent on temperature.

Thus, option (d) is correct. Molality remains independent of temperature.