Question

Air at a pressure of 101 kPa and a temperature of 20°C has its temperature raised to 4000 K in a constant-pressure process. Determine the composition of the air at this elevated temperature. Assume the air to initially consist of 3.76 mol N / mol O

Answer 1

Assuming air as an ideal gas.

For constant pressure process (isobaric).

Given = T1 = 293 K P =101 KPa

T2 =4000 K

Let take 1 m3 of Air.

So initial kmol of air N1= P1*V1/ R*T1

n air =101*1/8.314*293=0.04158kmol =

=41.58 mol

Mole fraction of N2=3.76/3.76+1=0.79

Mole fraction of O2 =0.21

At elevated temp

Moles of air = P2*V2/R*T2

P1=,P2

V1/T1 = V2/ T2

Or V2 =( V1/T1)*T2

V2=(1/293)*4000= 13.65 m³

As mole % = volume %

Moles of N2 = 13.65*0.79=10.78 kmol

Moles of O2 =2.865 kmol

Ratio N2/ O2 =10.78/2.865 =3.76 mol N2/mol O2

Thus composition remains same when gas is heated at constant pressure process.

Only volume changes.