Consider the solutions, 0.04 m urea [(NH2)2C=O)], 0.04 m AgNO3 and 0.04 m CaCl2. Which has (i) the highest osmotic pressure, (ii) the lowest vapor pressure, (iii) the highest boiling point?

Question

Consider the solutions, 0.04 m urea [(NH₂)₂C=O)], 0.04 m AgNO₃ and 0.04 m CaCl₂. Which has (i) the highest osmotic pressure, (ii) the lowest vapor pressure, (iii) the highest boiling point?

Is there an easier way to figure this out or do I have to write the equations and figure out the osmotic pressure, vapor pressure, and boiling point for each one?

Correct answer:

(i) 0.04 m CaCl2 (ii) 0.04 m CaCl2 (iii) 0.04 m CaCl2

Answer 1

Raising osmotic pressure, lowering vapor pressure and raising boiling point are all colligative properties so will depend on the number of particles in solution. For example,

π = iMRT where π = osmotic pressure, i = van't Hoff factor, M = molarity, R = gas constant and T = temperature in K

P_solution = (X_solvent)(Pº_solvent) where P_solution is vapor pressure of solution, X_solvent is mole fraction of solvent and Pº_solvent is vapor pressure of pure solvent

∆T = imK where ∆T = change in boiling point temperature, i = van't Hoff factor m = molality and K = boiling point constant of the solvent

So essentially, one only need look to see which solute produces the greatest number of particles when dissolved in the solvent, and that is the solute that will have the greatest effect.

urea is a non-electrolye and produces only 1 particle when dissolved

AgNO₃ is a strong electrolyte and produces Ag⁺ and NO₃^- when dissolved. Thus one has TWO particles.

CaCl₂ is a strong electrolyte and produces Ca²⁺ and 2Cl^- when dissolved. Thus one has THREE particles.

The answer is CaCl₂