The chapter “Solutions” is crucial for both CBSE and competitive exams because it connects measurable properties (like vapour pressure, boiling point rise, freezing point depression, and osmotic pressure) to composition and molecular-level effects (ideal behavior, Raoult’s/Henry’s laws, colligative properties, and dissociation/dimerization). Mastery of these concepts helps in solving both numerical and concept-based questions accurately and quickly.
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Marking
Q1. A student wants to lower the freezing point of of water by . How many grams of glucose (, ) must be dissolved? (Use and .)
Q2. 46 g of methanol () and 156 g of ethanol () are mixed to form an ideal solution at 298 K. Vapor pressures of pure methanol and ethanol are and . Using and , the mole fraction of methanol in the vapor phase is approximately:
Q3. A molal aqueous solution of causes a freezing point depression of . (.) Using and for , the degree of dissociation is:
Q4. At a binary solution contains component A at (very dilute) and component B at . A obeys Henry's law with , while B obeys Raoult's law with . The mole fraction of A in the vapour is closest to:
Q5. A student dissolves a fixed mass of an unknown solute in of water and measures the freezing point depression and the osmotic pressure at the same temperature. Using and (taking solution volume ≈ ), which statement is correct about determining the molar mass and degree of dissociation ?
Both and give independent equations so and can be determined uniquely.
alone is sufficient to determine uniquely for the given experiment.
Both and lead to algebraically equivalent expressions involving only the ratio for the same and solvent mass; hence cannot be determined without additional information about .
The osmotic pressure in this setup depends only on and not on or .
Q6. Pure benzene has vapour pressure at . When a non-volatile solute is dissolved in of benzene the vapour pressure of benzene above the solution at becomes . The number of moles of solute added is:
Q7. of a salt of type is dissolved in of water. The boiling point of the solution is found to increase by . (Take .) The degree of dissociation of the salt is closest to:
Q8. An ideal binary solution of volatile liquids A and B at has liquid mole fractions and . The vapour pressures of pure A and B at are and respectively. The mole fraction of A in the vapour phase is:
Q9. When of acetic acid (HA) is dissolved in of benzene the freezing point is depressed by . ( of benzene .) Acetic acid dimerizes in benzene: . If is the fraction of acid molecules that dimerize, the value of is closest to:
Q10. A binary ideal solution at has liquid composition . The total vapour pressure above the solution is and the mole fraction of A in the vapour phase is . Using Raoult's and Dalton's laws, the vapour pressures of pure A and pure B at , and , are approximately: