Question

The following data shows the variation of vapor pressure as a function of temperature.

T/K 323 353 394 413 433

P/mmHg 0.0127 0.0888 0.7457 1.845 4.189

Plot lnP vs 1/T and find its slope.

Determine what is T when P=760mmHg

Answer 1

Recall that in equilibrium; especially in vapor-liquid equilibriums, we can use Clasius Clapyeron combination equation in order to relate two points in the same equilibrium line.

The equation is given as:

ln(P2/P1) = -dHvap/R*(1/T2-1/T1)

Where

P2,P1 = vapor pressure at point 1 and 2

dH = Enthalpy of vaporization, typically reported in kJ/mol, but we need to use J/mol

R = 8.314 J/mol K

T1,T2 = Saturation temperature at point 1 and 2

Graph:

ln(P) = -7362.8*1/T+ 18.427

ln(760) = -7362.8*1/T+ 18.427

Solve for T

ln(760) - 18.427= -7362.8*1/T

T = 7362.8/11.79 = 624.49 K

T = 624.49 K for P = 760