The increasing order of osmotic pressure of 0.1 M aqueous solution of the following is:
(A) \( MgCl_2 \)
(B) \( NaCl \)
(C) \( NH_2CONH_2 \)
(D) \( K_3[Fe(CN)_6] \)
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → (C), (B), (A), (D)
Osmotic pressure is a colligative property: it depends on the number of solute particles, not on their nature. For a solution of an electrolyte the van’t Hoff factor i counts the particles each formula unit gives:
\( \displaystyle \pi = iCRT \)
All four solutions are 0.1 M at the same temperature, so \( \pi \) is proportional to i. Working out i from the dissociation:
(A) \( \ce{MgCl2 -> Mg^2+ + 2Cl-} \): 1 + 2 = 3 ions, i = 3
(B) \( \ce{NaCl -> Na+ + Cl-} \): 1 + 1 = 2 ions, i = 2
(C) \( \ce{NH2CONH2} \) (urea) is a non-electrolyte and does not dissociate: 1 particle, i = 1
(D) \( \ce{K3[Fe(CN)6] -> 3K+ + [Fe(CN)6]^3-} \): 3 + 1 = 4 ions (the complex ion stays intact), i = 4
Shortcut: Urea gives the fewest particles and \( \ce{K3[Fe(CN)6]} \) the most, so the order starts with (C) and ends with (D); only Option (3) does this.