Question

At 50.14 K a substance has a vapor pressure of 258.9 torr. Calculate its heat of vaporization in kJ/mol it if has a vapor pressure of 161.2 torr at 277.5 K.

Answer 1

Answer – Given, vapor pressure P₁ = 258.9 torr, T₁ = 50.14 K

T₂ = 277.5 K , vapor pressure, P₂ = 161.2 torr , ΔH_vap = ? kJ/mol

According to Clausius Clapeyron equation –

ln P₁/ P₂ = ΔH_vap / R *( 1/T₂ -1/T₁)

ln 258.9 torr / 161.2 torr = ΔH_vap / 8.314 J.mol^-1.K^-1 * ( 1/277.5 K – 1/50.14 K)

0.474 = ΔH_vap / 8.314 J.mol^-1.K^-1 * -0.0163 K

ΔH_vap= 0.474 * 8.314 / -0.0163 K

          = -241.1 J/mol

          = -0.241 kJ/mol