Look up the vapor pressure of water at 25 C and look up its enthalpy of vaporiza

Question

Look up the vapor pressure of water at 25 C and look up its enthalpy of vaporization. Using the Clausius-Clapeyron equation, estimate the vapor pressure of water at 75 C in Torr. Now look up the measured vapor pressure at 75 C. How good is your estimate? List the approximations that you made to make the estimate. Vapor pressures are often tabulated according to the Antoine equation. For water, the equation is log_10P = 5.08 - (1660/(T - 45.6)) between 344 and 373 K, where P is in bar and T is in Kelvin. Calculate the enthalpy of vaporization in kJ/mol in this temperature range from this equation, assuming it is constant.

Explanation / Answer

Vapor pressure is given by

Log P(bar)= 5.08-1660/(T-45.6)

At 344 K,

Log P = 5.08- 1660/(344-45.6), P= 0.33 bar,

Let this be P1= 0.33 bar

At 373 K

Log P= 5.08- 1660/(373-45.6), P:= 1.022 bar. Let this be P2= 1.022 bar

From Classius Clayperon equation

Ln(P2/P1) =(deltaH/R)*(1/T1-1/T2)

Where deltaH= Latent heat of vaporization

Ln(1.022/0.33)= (DeltaH/8.314)*(1/344-1/373)

deltaH= 41712 J/mole= 41.712 Kj/mole

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