Question

The concentration of a solution can be expressed in Molarity, Normality, Formality and Molality. Among them, which mode of expression is most accurate for all the conditions?
(A) Molarity
(B) Formality
(C) Normality
(D) Molality

Answer 1

Hint: Normality, Molarity and Formality include volume of solution in its formula. Molality expresses the amount of solution in weight. Different conditions may include potential changes into several physical parameters like temperature and pressure.

Complete answer:
Now, all conditions mean the quantity we use to express the concentration needs to be stable with the changes in physical parameters like temperature and pressure.
The expression of Molarity, Formality and Normality involves volume of solution. So, we can say that the value of Molarity, Formality and Normality will change with changes in temperature because the volume of solution will also change with changes in temperatures.
But Molality does not involve volume of solution in its expression, so it will not be affected by changes in temperature and pressure. So, it can be said that Molality will give the most accurate concentration of solution in all the conditions.

So., the correct answer of this question is (D) Molality.

Additional Information:
We can say that as temperature changes, the volume of solution or any liquid should also change is a universal law which is known as Charles law. It is expressed as \[\dfrac{V}{T} = k\] where V is volume of the liquid, T is temperature of liquid and k is a constant.
Here are the formulas of Molarity, Normality, Formality and Molality.
\[Molarity = \dfrac{{{\text{Moles of solute}}}}{{{\text{Volume of solution}}}}\]
\[Normality = \dfrac{{{\text{Normals of solute}}}}{{{\text{Volume of solution}}}}\]
\[Formality = \dfrac{{{\text{Formula weight of solute}}}}{{{\text{Volume of solution}}}}\]
\[Molality = \dfrac{{{\text{Weight of solute}}}}{{{\text{Weight of solution(kg)}}}}\]

Note:
Do not think that as we use molarity most often, it is the most accurate way of describing the concentration of any solution. Note that change in temperature and pressure do not have any effect on the mass of the compound.