0.01 molal aqueous solutions of strong electrolytes \(NaCl\), \(KCl\), \(MgSO_4\) and \(K_2SO_4\) are prepared. Which of the following statements is correct regarding their osmotic pressure? (Assume complete dissociation of solute particles)
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → \(K_2SO_4\) has highest osmotic pressure
Osmotic pressure is a colligative property: \( \pi = iCRT \). All four solutions have the same concentration (0.01 m) and are at the same temperature, so the one with the largest van’t Hoff factor i (number of particles per formula unit) has the highest osmotic pressure.
NaCl → \( \ce{Na^{+}} \) + \( \ce{Cl^{-}} \): i = 2
KCl → \( \ce{K^{+}} \) + \( \ce{Cl^{-}} \): i = 2
\( \ce{MgSO4} \) → \( \ce{Mg^{2+}} \) + \( \ce{SO4^{2-}} \): i = 2
K2SO4 → 2K+ + SO42−: i = 3
\( \ce{K2SO4} \) gives the most particles, so its osmotic pressure is the highest.
Why the other options are incorrect
Option (1) All of them exhibit equal osmotic pressure — Incorrect. \( \ce{K2SO4} \) gives 3 ions per unit and the others only 2.
Option (2) Osmotic pressure of all of them are different — Incorrect. NaCl, KCl and \( \ce{MgSO4} \) all have i = 2, so their osmotic pressures are equal.
Option (3) \(MgSO_4\) has highest osmotic pressure — Incorrect. \( \ce{MgSO4} \) gives only 2 ions; the charges on the ions do not change the particle count.
NCERT (2026-27 edition):
Abnormal Molar Masses – “… Table 1.4 depicts values of the factor, i for several strong electrolytes. For KCl, NaCl and MgSO4, i values approach 2 as the solution becomes very dilute. As expected, the value of i gets close to 3 for K2SO4.”