# standard enthalpy vap. of acetic anhydride ((CH3CO)2O)=45.7 kJ/mol. normal BP is 139.6 C vap. pressure of acetic anhydride at 125.6°C?

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- anonymousLv 79 months agoFavourite answer
Use the Clausius-Clapeyron equation. Temperatures need to be in Kelvin. The normal boiling point is the temperature at which a liquid boils at 1 atmosphere of pressure.

Clausius-Clapeyron equation:

ln(P₂/P₁) = (∆H°vap / R) x [(1/T₁) - (1/T₂)]

P₁ = 1 atm

T₁ = 139.6°C = (273.15 + 139.6) K = 412.75 K

P₂ = ?

T₂ = 125.6°C = (273.15 + 125.6) K = 398.75 K

∆H°vap = 45.7 kJ/mol = 45700 J/mol

R = 8.314 J/(mol•K)

Omitting the units to avoid clutter,

ln(P₂/P₁) = (∆H°vap / R) x [(1/T₁) - (1/T₂)]

ln(P₂ / 1) = (45700 / 8.314) x [(1/412.75) - (1/398.75)]

ln(P₂) = -0.46757

P₂ = e^(-0.46757)

P₂ = 0.627 atm …….. to 3 sig figs

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