Solutions are fundamental to both board and competitive exams because many key concepts—colligative properties (freezing point depression, boiling point elevation, osmotic pressure), Raoult’s law/ideal mixtures, and electrolytes/association/dissociation—directly test how particle number and thermodynamics connect to measurable changes in temperature and pressure.
20
Minutes
15
Questions
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Marking
Q1. A 0.500 g sample of a non-electrolyte solute is dissolved in 10.00 g of water. The freezing point of the solution is lowered by . Given and where is molality, the molar mass of the solute is:
Q2. Two volatile liquids A and B form an ideal solution at . Vapour pressures of the pure components are and . A solution contains mol A and mol B. Using Raoult's law and , the mole fraction of A in the vapour phase is:
Q3. A sample of NaCl of mass is dissolved in of water. The observed freezing point depression is . Given and for NaCl (where is degree of dissociation) and , the value of is:
Q4. An ideal binary mixture contains volatile liquids A and B with and . Initially there are moles of A and mole of B. At equilibrium the vapour in contact with the liquid is formed and mole of that vapour (with the equilibrium vapour composition) is removed. After removal, the remaining liquid has mole fractions and . The mole fraction of A in the remaining liquid is:
Q5. Acetic acid in benzene dimerizes: . A solution prepared by dissolving of acetic acid (molar mass ) in benzene shows a freezing point depression of . Given and for dimerization the van't Hoff factor , the degree of association is approximately:
Q6. A solution is prepared by dissolving of a non-volatile solute in of benzene. The freezing point of benzene is lowered by . Given and using , the molar mass of the solute (in ) is closest to:
Q7. A binary ideal liquid mixture contains of A () and of B (). At the temperature of interest the vapour pressures of pure components are and . Assuming ideal behaviour (Raoult's law), the mole fraction of A in the vapour phase is:
Q8. of (molar mass ) is dissolved in of water. The observed freezing point depression is . Given and assuming dissociates as with degree of dissociation (so ), the value of (approx) is:
Q9. Assertion (A): For a binary ideal solution at equilibrium, the component with the higher pure-component vapour pressure will always have a larger mole fraction in the vapour phase than the other component.
Reason (R): Raoult's law gives and the vapour-phase mole fraction is .
Which of the following is correct?
Both A and R are true and R is the correct explanation of A.
Both A and R are true but R is not the correct explanation of A.
A is true but R is false.
A is false but R is true.
Q10. of acetic acid (molar mass ) is dissolved in of benzene; the observed freezing-point depression is . Given and that acetic acid associates as , the fraction of acetic acid molecules present as dimers is closest to:
Q11. A sample of a non-volatile, non‑electrolyte solute (molar mass ) is dissolved in of water. Using and , the boiling point of the solution (assume ideal behaviour) is closest to:
Q12. A sample of a non‑volatile solute is dissolved in of water at . The vapour pressure of pure water at is and that of the solution is . Assuming ideal behaviour and no dissociation, the molar mass of the solute (use ) is approximately:
Q13. An ideal binary solution at has mole fractions and . The vapour pressures of pure components are and . Using Raoult's law, the mole fraction of A in the vapour phase above the solution is:
Q14. Assertion (A): For solutions in benzene at , acetic acid causes a larger freezing point depression than ‑hexane.
Reason (R): Acetic acid dimerizes in benzene, reducing the effective number of solute particles.
Both A and R are true and R is the correct explanation of A.
Both A and R are true but R is not the correct explanation of A.
A is true but R is false.
A is false but R is true.
Q15. A student measures the freezing point depression and the osmotic pressure of a dilute aqueous solution of an electrolyte at temperature and attempts to determine both the molar mass of the solute and its degree of dissociation from these two measurements alone. Which statement is correct?
It is possible because yields while yields independently.
It is not possible because both and depend on the same combination (e.g. and ), so they provide only one independent relation; an additional independent measurement (e.g. conductivity) is required to separate and .
It is possible only if the electrolyte dissociates into exactly two ions, since then and the two equations become independent.
It is not possible because freezing point depression is independent of the number of particles while osmotic pressure is not.