The osmotic pressure of the equimolar solutions of the following at the same temperature will increase in order
(A) \( NaCl \)
(B) \( K_2SO_4 \)
(C) \( K_3[Fe(CN)_6] \)
(D) \( NH_2CONH_2 \)
Choose the correct answer from the options given below.
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → (D), (A), (B), (C)
Osmotic pressure is a colligative property: it depends on the number of solute particles, not on their nature. For an electrolyte the van’t Hoff factor i (the number of particles each formula unit gives) enters the equation:
\( \displaystyle \pi = iCRT \)
For equimolar solutions at the same temperature, C and T are the same, so the osmotic pressure is proportional to i:
(A) NaCl: \( \ce{NaCl -> Na^+ + Cl^-} \), 2 ions, i = 2.
(B) \( \ce{K2SO4} \): \( \ce{K2SO4 -> 2K^+ + SO4^2-} \), 2 + 1 = 3 ions, i = 3.
(C) \( \ce{K3[Fe(CN)6]} \): \( \ce{K3[Fe(CN)6] -> 3K^+ + [Fe(CN)6]^3-} \), 3 + 1 = 4 ions (the complex ion stays intact), i = 4.
(D) \( \ce{NH2CONH2} \) (urea): a non-electrolyte; it does not dissociate, 1 particle, i = 1.
In increasing order of osmotic pressure:
\( \displaystyle \text{urea}\,(1) \lt \ce{NaCl}\,(2) \lt \ce{K2SO4}\,(3) \lt \ce{K3[Fe(CN)6]}\,(4) \)
Shortcut: Urea, the non-electrolyte, must come first and \( \ce{K3[Fe(CN)6]} \), with four ions, last. Only Option (4) starts with (D) and ends with (C).