Which of the following pairs of solutions are isotonic?

The correct answer is Option (3) → 0.1 M \(Ca(NO_3)_2\) and 0.1 M \(Na_2SO_4\)

Two solutions are isotonic when they have the same osmotic pressure at the same temperature. Since \( \pi = iCRT \), this needs the same value of i × C, that is, the same total concentration of solute particles. All the solutions here are 0.1 M, so the pair must have the same van’t Hoff factor.

    \( \displaystyle \ce{Ca(NO3)2 -> Ca^2+ + 2NO3^-}:\ 1 + 2 = 3 \text{ ions},\ i = 3 \)

    \( \displaystyle \ce{Na2SO4 -> 2Na+ + SO4^2-}:\ 2 + 1 = 3 \text{ ions},\ i = 3 \)

Both give \( iC = 3 \times 0.1 = 0.3 \) M of particles, so 0.1 M \( \ce{Ca(NO3)2} \) and 0.1 M \( \ce{Na2SO4} \) are isotonic.

NCERT (2026-27 edition):
Osmosis and Osmotic Pressure – “… Two solutions having same osmotic pressure at a given temperature are called isotonic solutions. When such solutions are separated by semipermeable membrane no osmosis occurs between them.”