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.
Which of the following statements is correct?
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → All are correct statements
Molality is a way of expressing concentration that uses the mass of the solvent, not the volume of the solution. It counts the moles of solute present for every kilogram of solvent.
Each of the first three statements follows from this definition:
Option (1) A molal solution is one that contains one mole of solute in 1000 g of solvent — Correct.
A 1 molal (1 m) solution has a molality of 1 mol per kg of solvent, so 1 mol of solute is dissolved in 1000 g (1 kg) of solvent.
The amount is per kilogram of solvent, not per litre of solution; “per litre of solution” describes a 1 molar (1 M) solution.
Option (2) Molality does not change with temperature — Correct.
Molality is built only from the moles of solute and the mass of solvent, and neither changes on heating or cooling.
Molarity uses the volume of the solution, which expands on heating, so molarity falls as the temperature rises while molality stays the same.
Option (3) Unit of molality is mol kg–1 — Correct. Moles of solute divided by mass of solvent in kg gives the unit:
\( \displaystyle \text{Molality}=\frac{\mathrm{mol}}{\mathrm{kg}}=\mathrm{mol\,kg^{-1}} \)
It is written with the symbol m, so a 0.5 molal solution is 0.5 m or 0.5 mol kg–1.
NCERT (2026-27 edition):
1.2 Expressing Concentration of Solutions
