Question

Two solutions, each of 100 mL having 4 g glucose and 10 g sucrose respectively. Amount of urea should be added to one of them in order to make them isotonic.

• A. 0.4218 g urea in glucose solution
• B. 0.4218 g urea in sucrose solution
• C. 0.2268 g urea in glucose solution
• D. 0.2268 g urea in sucrose solution

Answer 1

The correct option is A 0.4218 g urea in glucose solution

Osmotic pressure is a colligative property. Two solutions are isotonic when they have same osmotic pressure. They will have equal number of moles solute particles per unit volume of solution. Urea, glucose and sucrose do not undergo ionization.
Urea, glucose and sucrose have molar masses of $60$ g/mol, $180$ g/mol and $342$ g/mol respectively. Number of moles is the ratio of the mass to molar mass.
Amount of urea that should be added to solution having $4$ g glucose in order to make them isotonic is $[\frac{10}{342}-\frac{4}{180}]\times 60=0.4218$ g .