Phase Transition Chemistry - Determine Boiling Point

Question

Determine the normal boiling point of a substance whose vapor pressure is

55.1 mm Hg at 35°C and has a ΔHvap of 32.1 kJ/mol.

Answer 1

Use the Clausius-Clapeyron equation: ln(P2/P1) = -ΔHvap/R * (1/T2 - 1/T1).

You have the values for P2, P1, -ΔHvap, R (constant), and T1. You want to solve for the other temperature, T2. Remember the definition of boiling point - the temperature at which vapor pressure is equal to the atmospheric pressure (which you can assume is 1 atm = 760 mmHg).

Make sure your units are converted properly.

Happy to walk through the step by step solution if you book time with me.

Answer 2

The relationship between the vapor pressure of a substance and the temperature is seen in the Clausius Clapeyron equation

ln(P₂/P₁) = -∆H_vap/R x (1/T₂ - 1/T₁)

P1 = 55.1 mm Hg

P2 = 760 mm Hg (the normal atmospheric pressure)

VH_vap = 32.1 kJ/mol

R = 8.314 J/molK = 0.008314 kJ/molK (change units to agree with units for ∆H_vap, i.e. kJ

T1 = temperature in K associated with P1 = 35.C + 273 = 308K

T2 = temperature in K associated with P2 = ?

Solving for T2 we have the following:

ln(760 / 55.1) = -(32.1 / 0.008314) (1/T2 - 1/308)

2.62 = -3861(1/T2 - 0.003247)

2.62 = -3861 / T2 + 12.54

-9.92 = -3861 / T2

T2 = 389K

T2 = 116ºC