Using Clausius-Clapeyron relation: Ln(P2/P1) = (DH/R)*(1/T1-1/T2) where DH reads Delta H, and is the Hvap you are looking for when P1641,P2=1atm, T1=81.2C, and T2=99C. Solution Using Clausius-Clapeyron relation: Ln(P2/P1) = (DH/R)*(1/T1-1/T2) where DH reads Delta H, and is the Hvap you are looking for when P1641,P2=1atm, T1=81.2C, and T2=99C..