Which of the following is the residual molar enthalpy of ethane (to two significant figures) at T = 500 K and P = 3 MPa, as calculated using the Lee-Kesler approach?
A. -0.4 J/mol
B. -1000 J/mol
C. -1700 J/mol
D. -2400 J/mol
E. None of these are correct.
A. Incorrect. The graph ordinate is HR/RTc, which must be converted to just HR.
B. Correct. The second term on the right hand side in Equation 7.81 is essentially zero from Figure 7-17, so the acentric factor is not actually needed.
C. Incorrect. You multiplied by T to solve for HR. Recheck Equation 7.81.
D. Incorrect. You probably used the wrong graphs! Figures 7-14 and 7-15 are for the compressibility factor only. Figures 7-16 and 7-17 are for the enthalpy residual.
E. One of these four is correct, though it is possible we have just read the graphs slightly differently. Try the calculation using our value of HR/RTc = -0.4 from Figure 7-16.
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