A compound has TC = 500 K, PC = 5 MPa and ? = 0.4. The compound is known to have Psat = 2 kPa and ?Hvap = 30 kJ/mol at T = 250 K. No other data is available. Which of the following is the most accurate way to estimate the boiling point at a pressure of 1.5 kPa?
A. The Clausius-Clapeyron equation
B. The shortcut equation
C. The Antoine equation
D. The van der Waals equation of state
E. All of these are possible but none will provide an estimate accurate enough to be useful.
A. Correct. Clausius-Clapeyron directly relates two boiling temperatures and pressures with the heat of vaporization. Since the known data point is at Tr=0.5 and the unknown boiling point will be at an even lower temperature, this method is preferred over shortcut.
B. Incorrect. The shortcut equation can be used here, but at least one other method is preferred this far from the critical point.
C. Incorrect. We do not have the three Antoine constants (A, B and C) required.
D. Incorrect. We can find the two van der Waals constants (a and b) since we know the critical point, but the van der Waals equation does not typically model vapor pressures at all accurately.
E. Incorrect. At least one is impossible, and at least one is reasonable.
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