# PA7 - Problem Analysis#7 Material and Energy Balance(March...

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Problem Analysis #7: Material and Energy Balance (March 16) Problem 1. Heat capacity and enthalpy The heat capacity of substance is often expressed as a function of temperature using a polynomial: C p = a + bT + cT 2 + dT 3 The following values have been reported for water vapor for T in C and Cp in J/mol× C: a = 33.46 b = 0.688×10 -3 c = 0.7604×10 -5 d = -3.593×10 -9 a. Calculate the enthalpy of water vapor at 1500 C in relative to water vapor at 25 C. b. Calculate the enthalpy of water vapor at 1500 C in relative to liquid water at 0 C if H v of H 2 O at this temperature is 2500kJ/kg. [ Note: We learnt in class that C p does not vary greatly in narrow temperature ranges (try to prove this statement by calculating C p at different T values using the above polynomial). Therefore, unless stated otherwise, it can be assumed that C p is a constant in many problems.] Problem 2. Enthalpy of humid air Enthalpy, H = U + pV, has no absolute value; only changes in enthalpy can be calculated. To simplify H and, therefore,
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