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Free questions · JEE Advanced · Chemistry

Liquids and Intermolecular Interactions: questions with solutions

3 free questions from this chapter (the full set has 30). Marking: Multi-correct: +4 all correct, +1 per correct option if none wrong, −2 if any wrong · Numerical: +4 / 0 · Matching: +3 / −1.

Question 1Level 2 · ExamSingle correct
The normal boiling points of HO, HF, HCl and HI are to be compared. Which order (highest first) is correct? Exactly one option is correct.
AHF > HO > HI > HCl
BHO > HF > HCl > HI
CHO > HF > HI > HCl
DHO > HI > HF > HCl
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Answer: C

HO and HF are hydrogen bonded, so both boil far above the heavier hydrogen halides. Water boils higher than HF (373 K vs 293 K) because each HO can take part in about four hydrogen bonds (two O–H donors, two lone pairs) while each HF effectively forms two (one H donor), even though a single F–HF bond is stronger. Between HCl (188 K) and HI (238 K) there is no significant hydrogen bonding. HCl has the larger dipole moment, but HI is much larger and more polarisable, so its London dispersion forces dominate and it boils higher. Order: HO > HF > HI > HCl, option C. Difficulty: Hard
Question 2Level 3 · AdvancedOne or more correct
A pure liquid and its vapour are at equilibrium in a closed container at constant temperature. Which of the following statements is/are correct? One or more options may be correct.
AThe equilibrium vapour pressure does not depend on the amount of liquid present or on the area of the liquid surface.
BAt equilibrium, the rate of evaporation equals the rate of condensation.
CIf the volume of the container is increased isothermally while some liquid still remains, the equilibrium vapour pressure decreases.
DTwo liquids X ( = 30 kJ mol) and Y ( = 40 kJ mol), each with constant , have equal vapour pressures at 300 K. At 320 K, the vapour pressure of Y is greater than that of X.
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Answer: A, B, D

(A) Correct. The equilibrium vapour pressure depends only on the nature of the liquid and the temperature. More surface area increases the rates of evaporation and of condensation equally. (B) Correct. The equilibrium is dynamic, with equal rates of evaporation and condensation. (C) Incorrect. If the volume is increased while liquid remains, more liquid evaporates until the pressure is back to the same vapour pressure. (D) Correct. From , the increase is larger for larger . Starting from equal pressures at 300 K: for X, ratio = = 0.752, a factor of 2.12. For Y, 1.002, a factor of 2.72. So at 320 K. Answer: A, B, D. Difficulty: Very hard
Question 3Level 2 · ExamSingle correct
Which order of normal boiling points of the group 15 hydrides NH, PH, AsH and SbH (highest first) is correct? Exactly one option is correct.
ANH > SbH > AsH > PH
BSbH > NH > AsH > PH
CSbH > AsH > PH > NH
DNH > PH > AsH > SbH
Show answer and solution

Answer: B

From PH to SbH the molecules get larger and more polarisable, so London dispersion forces and boiling points increase: PH (185 K) < AsH (211 K) < SbH (256 K). NH is hydrogen bonded, which raises its boiling point (240 K) above those of PH and AsH. It is still not enough to beat the very strong dispersion forces of the much heavier SbH. Order: SbH > NH > AsH > PH, option B. Difficulty: Hard

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