Question

The vapor pressure of ice is 4.579 torr at 0.0ºC and 0.766 torr at –20ºC . Estimate the vapor pressure at –16.3ºC.

Answer 1

According to the Clausius-Clapeyron Equation Vapor pressure is given by

     P1    DHsub    1     1
ln (---) = ----  (--- - ----)
     P2R      T2    T1

_{T1 = 273K, T2 = -20+273 = 253 K,}

_{Thus DHsub =}R ln (P₂₇₃ / P₂₅₃) /(1/253 - 1/273)

== 8.3145*ln(4.579/0.766) / (1/268 - 1/273)

== 51341 J mol^-1

^{Vapor pressure at -16.3 C ie 256.7 K is}

ln (P_256.7 / P₂₇₃ ) = (DHsub/R ) * (1/T2 - 1/T1)

ln (P_256.7 / P₂₇₃ ) = (51341/8.3144)* (1/273- 1/256.7)

P256.7 = 1.0889 torr Ans