5. Water has a standard boiling point of 99.60 °C and a vapor pressure of 42.23

Question

5. Water has a standard boiling point of 99.60 °C and a vapor pressure of 42.23 torr at 35.0 °C Caohg he Clauslis-Ciapeyroam equation. (Equation 5.21) Clapeyron equation you derived in question 4. Use ½-30.517 L mol and the temperature and pressure of the standard boiling point to calculate Z. (c) Compare your answers from parts a and b to the literature value for the enthalpy of vaporization of water. (Cite your source for the literature value.) What improvement is gained by not assuming ideal gas behavior?

Explanation / Answer

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

Therefore, we need at least 4 variables in order to solve this.

Substitute all known data:

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

Change negative signs

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

ln(760/42.23) = Hv/(8.314)*(1/(35+273) - 1/(99.6+273))

Hv = ln(760/42.23) *8.314/(1/(35+273) - 1/(99.6+273))

Hv = 42687.14 J/mol

HV = 42.687 kJ/mol

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