An ideal gas mixture consists of 35% helium and 65% argon gases by mass. The mixture is now expanded isentropically in a turbine from 400°C and 1.2 MPa to a pressure of 150 kPa. The mixture temperature at turbine exit is
A)174°C
B)20°C
C)99°C
D)148°C
E)256°C

Question

Quizplus · Accepted Answer

Solved by EES Software. Solutions can be verified by copying-and-pasting the following lines on a blank EES screen. 
T1=400+273"K"
P1=1200 "kPa"
P2=150 "kPa"
mf_He=0.35
mf_Ar=0.65
k1=1.667
k2=1.667
"The specific heat ratio k of the mixture is also 1.667 since k=1.667 for all componet gases"
k_mix=1.667
T2=T1*(P2/P1)^((k_mix-1)/k_mix)-273
 
"Some Wrong Solutions with Common Mistakes:"
W1_T2 = (T1-273)*(P2/P1)^((k_mix-1)/k_mix) "Using C for T1 instead of K" 
W2_T2 = T1*(P2/P1)^((k_air-1)/k_air)-273; k_air=1.4 "Using k value for air"
W3_T2 = T1*P2/P1 "Assuming T to be proportional to P"

An Ideal Gas Mixture Consists of 35% Helium and 65