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.
Assertion (A): In a 10% (V/V) solution of ethanol in water, 10 mL of ethanol is present in every 100 mL of the solution.
Reason (R): The concentration of a solution of one liquid in another liquid is commonly expressed as volume percentage.
In the light of the above statements, choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Both (A) and (R) are true but (R) is not the correct explanation of (A).
Assertion (A): In a 10% (V/V) solution of ethanol in water, 10 mL of ethanol is present in every 100 mL of the solution. — True. 10 mL of ethanol is dissolved in water so that the total volume of the solution is 100 mL (not 10 mL of ethanol in 100 mL of water).
Reason (R): The concentration of a solution of one liquid in another liquid is commonly expressed as volume percentage. — True. Solutions containing liquids are commonly expressed in this unit, for example the 35% (V/V) ethylene glycol antifreeze used in cars.
Why R does not explain A?
R only tells where volume percentage is commonly used; it does not tell what a 10% (V/V) solution contains.
The correct reason for A is the definition of volume percentage: the volume of the component per total volume of the solution × 100, so 10% (V/V) means 10 mL of ethanol in 100 mL of solution.
NCERT (2026-27 edition):
1.2 Expressing Concentration of Solutions – “(ii) Volume percentage (V/V): The volume percentage is defined as:”
\( \displaystyle \text{Volume \% of a component} = \frac{\text{Volume of the component}}{\text{Total volume of solution}} \times 100 \qquad (1.2) \)
“For example, 10% ethanol solution in water means that 10 mL of ethanol is dissolved in water such that the total volume of the solution is 100 mL. Solutions containing liquids are commonly expressed in this unit. For example, a 35% (v/v) solution of ethylene glycol, an antifreeze, is used in cars for cooling the engine.”