Read the passage carefully and answer the questions.
The osmotic pressure is one of the four colligative properties which depends on the number of particles of solute in the solution, irrespective of their nature. The osmotic pressure is equal to the product of concentration, gas constant and temperature. It is the most commonly used property for the determination of the molar mass of biomolecules. In many situations, the molecules get dissociated or associated when they are dissolved in a solvent, thereby changing the number of molecules in the solution. As a result, an abnormal molecular mass is obtained. The extent of dissociation or association is given by van't Hoff factor (i), which is the ratio of normal molar mass to abnormal molar mass. It is related with the degree of association or ionization. The introduction of van't Hoff factor in the osmotic pressure equation modifies the equation.
Which one of the following has minimum osmotic pressure at 298 K?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → 1 M glucose solution.
Osmotic pressure is a colligative property: it depends on the number of solute particles, not on their nature. An electrolyte gives more particles than its formula units, which the van’t Hoff factor i takes into account:
\( \displaystyle \pi = i\,C\,R\,T \)
At the same temperature, R and T are common to all four solutions, so only the product i × C has to be compared. The value of i comes from the dissociation (complete dissociation assumed):
KCl: \( \ce{KCl -> K+ + Cl-} \): 2 ions, i = 2; iC = 2 × 3 = 6 M.
Glucose: a non-electrolyte, does not dissociate: 1 particle, i = 1; iC = 1 × 1 = 1 M.
Urea: a non-electrolyte: i = 1; iC = 1 × 2 = 2 M.
\( \ce{K2SO4} \): \( \ce{K2SO4 -> 2K+ + SO4^2-} \): 3 ions, i = 3; iC = 3 × 2 = 6 M.
The 1 M glucose solution has the smallest effective particle concentration, so it has the minimum osmotic pressure.