Question

Enthalpy change for a process of 100 lbmol of air from 100F to 700F (BTU) is:

a) 429100

b) 375100

c) 3751

d) 4291

Answer 1

Specific heat capacity of air = C_{​​​​​​p} = 0.241 BTU/lb °F

Amount of air = 100 lbmol

Molecular weight of air = 29 lb/lbmol

Amount of air =m= 100 lbmol × 29 lb/lbmol = 2900 lb

Change in temperature = ∆T = ( 700° F - 100° F) = 600° F

Enthalpy change = m C_{​​​​​​p} ∆T = 2900 lb × 0.241 BTU/lb °F × 600 °F = 419340 BTU

Actual answer may vary a bit due to some approximations in the C_{​​​​​​p} and molecular weight of air.

Correct answer is option A.