The boiling point of 0.2 mol kg-1 solution of X in water is greater than equimolal solution of Y in water. Which one of the following statements is true in this case?
- Molecular mass of X is less than the molecular mass of Y.
- Y is undergoing dissociation in water while X undergoes no change.
- X is undergoing dissociation in water.
- Molecular mass of X is greater than the molecular mass of Y.
Answer
(3) X is undergoing dissociation in water.
Solution
Elevation of boiling point is Tb = i Kb m.
Both solutions have the same solvent (Kb same) and the same molality (m = 0.2 mol kg-1), so Tb i.
Tb(X) > Tb(Y), i(X) > i(Y); hence X gives more particles in water, i.e. X undergoes dissociation. Molecular mass does not matter at equal molality.
Answer: (3)