1. a volume of 20 cu ft of air at pressure of 50 psig is heated and expanded to 30 cu ft and 40psig find the change in
the energy of air in joules.
(A) 35564 j
(B) 48219 j
(C) 123378 j
(d) 167278 j
V1 = 20 cft = 20 * 28.32/1000 = 0.5664 m^3 and V2 = 30 cft = 30 * 28.32/1000 = 0.8496 m^3
P1 = 50 psig + 14.7 = 64.7 psi = 64.7 * 6894.76 = 446091 Pa
P2 = 40 psig + 14.7 = 54.7 psi = 54.7 * 6894.76 = 377143.37 Pa
n * R * T = P2V2 – P1V1 = 377143.37 * 0.8496 – 446091 * 0.5664 = 67755.06
R = 8.314 J/mol K; Let n = 1 mole
1 * 8.314 * T = 67755.06  T = 67755.06/8.314 = 8149.51 K
Change in the internal energy of air = Cv * T = 20.76 * 8149.76 = 169189 J
Option (d) [Since that is the closest answer.]
2>A quantity of gas equal to 35 cu ft is to be expanded adiabatically. It has specific heat at constant pressure of 0.532
and constant volume of 0.408.what is the value of exponential power k?
(a) 0.124
(b) 0.217
(c) 0.94
(d) 1.30
The adiabatic index =  = Cp/Cv = 0.532/0.408 = 1.304
Option (d).