Tetrachloromethane, CCl₄, which is now known to be carcinogenic, was once used as a dry-cleaning solvent and is a volatile liquid still used in the manufacture of coolants. The enthalpy of vaporization of CCl₄ is 33.05 kJ · mol^–1, and its vapor pressure at 57.8°C is 405 Torr. What is the vapor pressure of tetrachloromethane at 25.0°C?

ANTICIPATE Vapor pressure increases with temperature, so you should expect the vapor pressure of CCl₄ to be lower at 25.0°C than at 57.8°C and therefore less than 405 Torr.

PLAN Substitute the temperatures (in kelvins) and the enthalpy of vaporization (in joules per mole) into the Clausius–Clapeyron equation to find the ratio of vapor pressures. Then substitute the known vapor pressure to find the one required. Note that the vapor pressure P₁ corresponds to the temperature T₁.

What should you assume? Assume that ΔH_vap° and ΔS_vap° are constant over the temperature range of interest and that the vapor behaves as an ideal gas (so that the Clausius–Clapeyron equation can be used).