Question

One mole each of urea, glucose and sodium chloride were dissolved in one litre of water Equal osmotic pressure will be produced by solutions of:
A.Glucose and sodium chloride
B.Urea and glucose
C.Sodium chloride and urea
D.None of these

Answer 1

To answer this question you should recall the concept of osmotic pressure. Sodium chloride dissociates into sodium and chloride ions in the aqueous solution while glucose and urea do not dissociate in water.

Complete step by step answer:
-The Van’t Hoff factor which is an important aspect of colligative properties is defined as the ratio of the concentration of particles formed when a substance is dissolved to the concentration of the substance by mass. In the case of non-electrolytic substances which do not dissociate in water, the value of $i$ is generally 1. But in the case of ionic substances, due to dissociation, the value of $i$ is equal to the total number of ions present in one formula unit of the substance.
-Osmotic pressure is a colligative property which depends upon the number of moles of solute present in the solution. The expression for osmotic pressure can be given by $\Pi = iCRT$ where $\Pi$ = Osmotic pressure, $i$ = Van’t Hoff Factor, $C$ = concentration, $R$ = Universal gas constant and $T$ =Temperature.
-Among the options we have: ${\mathbf{NaCl}}$, when dissolved in water, gives 2 moles of ions $\left( {\;{\mathbf{N}}{{\mathbf{a}}^ + }\;{\mathbf{and}}\;{\mathbf{C}}{{\mathbf{l}}^ - }} \right)$ on dissociation, as one mole of NaCl is dissolved.
$NaCl \to N{a^ + } + C{l^ - }\;{\text{ }}\; \Rightarrow \;\;i = 2$.
But urea and glucose do not ionise. They are non-electrolytes. So, the Van't Hoff factor of both urea and glucose will be $i$ = 1. Therefore, equal osmotic pressure will be equal in the case of urea and glucose only.

Hence, option B is correct.

Note:
In Association the Observed molar mass is greater than the predicted value and The value of the Van’t Hoff factor is less than one. The values of the colligative properties are lower than expected. Example: reduced boiling point and freezing point. While in Dissociation the observed value of molar mass is lesser than the normal value, The value of i is greater than one. Higher values of colligative properties are observed.