valence
Practice
States of matterNational 2020

Which liquid has the highest vapor pressure at 25 °C?

13% of national qualifiers got this right

Explanation

draft

Vapor pressure depends on the strength of intermolecular forces (mainly London dispersion forces for these nonpolar/weakly polar molecules). Weaker intermolecular attractions → more molecules escape to the gas phase → higher vapor pressure.

Dispersion forces increase with molecular size/polarizability (roughly with molar mass and number of electrons):

  • CHClX3\ce{CHCl3}: M≈119M \approx 119 g/mol
  • CFClX3\ce{CFCl3}: M≈137M \approx 137 g/mol, but F is small and only weakly polarizable, and this molecule has the smallest surface area/polarizability among the heavier halogens present
  • CClX4\ce{CCl4}: M≈154M \approx 154 g/mol
  • CBrClX3\ce{CBrCl3}: M≈198M \approx 198 g/mol (largest, most polarizable, strongest dispersion forces)

Comparing the four, CBrClX3\ce{CBrCl3} has the largest, most polarizable electron cloud (heaviest halogens), giving the strongest dispersion forces and lowest vapor pressure. CFClX3\ce{CFCl3} (Freon-11) contains a small, weakly polarizable F atom in place of one Cl, reducing the overall polarizability/surface area compared to CClX4\ce{CCl4} or CBrClX3\ce{CBrCl3}, and it also lacks the H-bond-like dipole enhancement seen in CHClX3\ce{CHCl3}. Experimentally, CFClX3\ce{CFCl3} (bp ≈ 24 °C) is the most volatile of the four, giving it the highest vapor pressure at 25 °C.

Thus the correct order of vapor pressure is CFClX3>CHClX3>CClX4>CBrClX3\ce{CFCl3} > \ce{CHCl3} > \ce{CCl4} > \ce{CBrCl3}, matching the trend of increasing molar mass/polarizability with decreasing vapor pressure.

A

Student assumes CHClX3\ce{CHCl3}'s slight polarity (dipole moment) makes it the most volatile, overlooking that its dispersion forces plus dipole-dipole interactions actually give it a higher boiling point and lower vapor pressure than CFClX3\ce{CFCl3}.

B

Correct: CFClX3\ce{CFCl3} has the weakest net intermolecular attractions among the four (small, weakly polarizable F atom lowers dispersion forces relative to the heavier all-Cl/Br analogs), giving it the lowest boiling point (~24°C) and thus highest vapor pressure at 25°C.

C

Student picks CClX4\ce{CCl4} reasoning that it's simplest/symmetric and nonpolar, ignoring that its larger polarizable Cl atoms (compared to CFClX3\ce{CFCl3}) create stronger dispersion forces and a higher boiling point.

D

Student mistakenly assumes greater halogen substitution/molar mass correlates with higher volatility, but heavier, more polarizable Br atom actually strengthens dispersion forces, making CBrClX3\ce{CBrCl3} the least volatile of the four.

Concept · Vapor pressure and intermolecular (dispersion) forces

USNCO 2020 National Part I exam, question 13. © American Chemical Society; reproduced for non-commercial practice.