Question

Consider the two solutions:
I: $0.5 M NaCl$ aqueous solution at $25^{\circ} C ; NaCl$ is complete ionized
II: $2.0 M C _6 H _5 COOH$ in benzene at $25^{\circ} C , C _6 H _5 COOH$ dimerizes to the full extent
Which of the following statement(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), (D)

$NaCl: 0.5 M (i=2) \Rightarrow C_{\text {eff }}=0.5 \times 2=1 M$
$C _6 H _5 COOH : 2.0 M \left(i=\frac{1}{2}\right) \Rightarrow C_{\text {eff }}=2 \times \frac{1}{2}=1 M$
a. $\pi=C_{\text {eff }} R T \Rightarrow \pi$ is same as $C_{\text {eff }}$ is same
b. Since both the solutions have different solvents, vapor pressure will be different
c. Solutions are isotonic
d. $\Delta T_{ f }=i K_{ f } \cdot m \Rightarrow \Delta T_{ f }$ for solution II will be more as $\left(K_{ f }\right)_{\text {benzene }}>\left(K_{ f }\right)_{\text {water }}$