Choose the aqueous solution with the highest vapor pressure. These are all solutions of nonvolatile solutes and you should assume ideal van't Hoff factors where applicable.

Question

A. 0.50 m urea [(NH2)2C=O)]

B. 0.50 m glycerin, C₃H₈O₃

C. 0.50 m C6H12O6

D. 0.50 m C12H22O11

E. They all have about the same vapor pressure.

Answer 1

Answer: E

Roult's law of non-volatile solutes is the main concept here. Roult's law summarized: With ideal gases, the pure solvent will always have higher vapor pressure than the corresponding solution with non-volatile solutes present. Mathematically, Roult's law can be expressed as: Vapor Pressure above the Solution = Mole Fraction of Solvent * Vapor Pressure of Pure Solvent. [mole fraction of solvent = mole solvent / total number of moles in solution]

The Vapor Pressure of a Solution is influenced by the mole fraction of solvent (ie how much solvent there is). A solution with a high concentration of non-volatile solute would mean that the solvent concentration would be lower than it would be compared to the pure solvent only, so the vapor pressure of the solution will be lowered with respect to the pure solvent.

With this question, first thing to notice is that the molar concentration of the non-volatile solutes given is the same for all species: 0.50 M. Looking at the Roult's Law equation, the mole fraction of the solvent will be equivalent for all the species since all the species has the same number of moles.

Second thing to notice about this question is the non-ionic nature of the compounds. All compounds given are non-ionic. This is in contrast to compounds like NaCl or KBr which are ionic in nature. Ionic compounds will dissociate into the corresponding electrolytes, whereas, non-ionic compounds will not. This will have a huge bearing on the mole fraction.

Example: 0.5M NaCl when dissolved in solvent will dissociate into 0.5M Na+ and 0.5MCl-. This fact comes into play when calculating the mole fraction as this will ultimately have a higher "total number of moles in solution" than when compared to a similar concentration of non-ionic compounds. 0.5M urea will remain 0.5M of urea as it does not dissociate into ions.