Question

Consider the two solutions:
I. 0.5 M $NaCl$ aqueous solution at ${25}^{o}C$; $NaCl$ is completely ionized.
II. 2.0 M $C_6{H}_{5}COOH$ in benzene at ${25}^{o}C;C_6{H}_{5}COOH$ dimerizes to the full extent.
Which of the following statements(s) is/are correct?

• A. Both the solutions display equal osmotic pressure
• B. Both have equal vapour pressure
• C. Solution $II$ is hypertonic
• D. Solution $II$ has greater depression in freezing point than solution $I$

Answer 1

Answer: (A), (B)

The correct options are
A Both the solutions display equal osmotic pressure
B Both have equal vapour pressure

The statements (A) and (B) are correct.
Both the solutions display equal osmotic pressure.
Both have equal vapour pressure.
Osmotic pressure and relative lowering of vapour pressure are colligative properties and depends on the number of solute particles.
I. 0.5 M NaCl aqueous solution at ${25}^{o}C$; $NaCl$ is completely ionized. 1 L of solution will contain $1L\times 0.5mol/L\times 2=1.0mol$ of particles as one molecule of $NaCl$ dissociate to give two ions (particles).
II. 2.0 M $C_6{H}_{5}COOH$ in benzene at ${25}^{o}C$; $C_6{H}_{5}COOH$ dimerizes to the full extent. 1 L of solution will contain $1L\times 2.0mol/L\times 0.5=1.0mol$ of particles as two molecules of $C_6{H}_{5}COOH$ associate to give one dimer (particles).
Thus, 1 L each of solution I and II will contain equal number of solute particles. Hence, they will have equal value of colligative property.