Question

Which of the following concentration terms is temperature independent?
(I) Molarity (II) Molality (III) Normality (IV) Mole fraction
(A) I & II
(B) I & III
(C) II only
(D) II and IV

Answer 1

Hint: The terms that are dependent upon the volume will change with changes in temperature as volume changes with changes in temperature. The mass or weight does not change with change in temperature.

Complete step by step solution:
We will take a look at the formulas of all the given concentration terms in order to find which of them are temperature independent.
- Note that the mass or weight of the solution does not alter the changes in temperature. Volume is a quantity that changes with temperature. So, the concentration term that involves volume will vary with changes in temperature.
(I) Molarity:
Molarity of the solute is the number of moles dissolved in a unit volume of the solvent. This can be expressed in the formula as
\[{\text{Molarity = }}\dfrac{{{\text{Weight of Solute}}}}{{{\text{Molecular weight of Solute}} \times {\text{Volume of solution}}}}\]
We can see that here, volume is involved in the formula. So, changes in temperature will affect the value of molarity.
(II) Molality:
It can be defined as the number of moles of solute dissolved in 1kg of a solvent. It formula can be given by
\[{\text{Molality = }}\dfrac{{{\text{Weight of solute}}}}{{{\text{Molecular weight of Solute}} \times {\text{Weight of solvent}}}}\]
Molality is not dependent upon any quantity that changes with temperature. So, it is temperature independent.
(III) Normality:
It is the number of normals of solute dissolved in unit volume of solvent. It formula is
\[{\text{Normality = }}\dfrac{{{\text{Weight of solute}}}}{{{\text{Equivalent weight of solute}} \times {\text{Volume of the solution}}}}\]
Normality depends upon the volume of the solution. Thus, it will vary with temperature changes.
(IV) Mole fraction:
It is the fraction of the moles of the solute to the total moles in the solution. It formula is
\[{\text{Mole fraction = }}\dfrac{{{\text{Moles of solute}}}}{{{\text{Total number of moles in solution}}}}\]
It will also not vary with changes in temperature as it is not dependent upon volume.
So, Molality and mole fraction will not change with changes in temperature.

Therefore, the correct answer is (D).

Note: Remember that Molality is the number of moles of solute dissolved in 1kg of a solvent. Many times we consider it as 1kg of the solution and this leads to errors. Note that molality is not affected by the change in temperature.