The concentration of a solution may be defined as the amount of solute present in the given quantity of the solution.
The concentration of solution may be expressed in several ways as discussed below:
Mass Percentage
The mass percentage of a component in a given solution is the mass of the component per 100 g of the solution.
\[\text{Mass percentage of a component = }\frac{\text{Mass of the component in the solution}}{\text{Total mass of the solution}} × 100\]
Molarity of a solution
The molarity of a solution is defined as the number of moles of the solute dissolved per litre of the solution.It is represented as ′M′. Mathematically,
\[Molarity = \frac{\text{Moles of solute}}{\text{Volume of the solution (in mL)}} × 1000\]
Molality of the solution
The molality of a solution is defined as the number of moles of the solute dissolved per 1000 g (or 1 kg) of the solvent.
Consider the following statements about the ways of expressing the concentration of a solution.
(A) Concentration in parts per million can be expressed as mass to mass, volume to volume and mass to volume.
(B) In a binary mixture of A and B containing nA and nB moles respectively, the mole fraction of A is nA/nB.
(C) In a given solution, the sum of all the mole fractions is unity.
(D) Describing a solution as dilute or concentrated is a quantitative description of its concentration.
Which of the statements given above are correct? Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → (A) and (C) only
The concentration of a solution can be described qualitatively (dilute or concentrated) or quantitatively, by terms such as parts per million and mole fraction that give the actual amount of solute.
(A) Concentration in parts per million can be expressed as mass to mass, volume to volume and mass to volume. — True. As with percentage, ppm can be taken as mass to mass, volume to volume or mass to volume.
(B) In a binary mixture of A and B containing nA and nB moles respectively, the mole fraction of A is nA/nB. — False. The moles of A are divided by the total moles of both components; nA/nB is only the mole ratio of A to B.
\( \displaystyle x_A = \frac{n_A}{n_A + n_B} \)
(C) In a given solution, the sum of all the mole fractions is unity. — True. x1 + x2 + … + xi = 1, so in a binary solution x2 = 1 − x1.
(D) Describing a solution as dilute or concentrated is a quantitative description of its concentration. — False. Dilute and concentrated are qualitative words; they do not give the amount of solute, which is why quantitative terms are needed.
NCERT (2026-27 edition):
1.2 Expressing Concentration of Solutions – “…For example, qualitatively we can say that the solution is dilute (i.e., relatively very small quantity of solute) or it is concentrated (i.e., relatively very large quantity of solute). But in real life these kinds of description can add to lot of confusion and thus the need for a quantitative description of the solution.”
“As in the case of percentage, concentration in parts per million can also be expressed as mass to mass, volume to volume and mass to volume.”
“For example, in a binary mixture, if the number of moles of A and B are nA and nB respectively, the mole fraction of A will be”
\( \displaystyle x_A = \frac{n_A}{n_A + n_B} \qquad (1.5) \)
“It can be shown that in a given solution sum of all the mole fractions is unity, i.e. x1 + x2 + … + xi = 1”