Question

Calculate AS for the system when the state of 3.00 mol of gas molecules for which Cp.m = R, is changed from 25 °C and 1.00 atm to 125 °C and 5.00 atm.

Answer 1

Solution:

Entropy change (ΔS) is calculated by Cp as,

ΔS = n Cp ln T2/T1 - n R ln P2/P1

Where,

Cp = 5/2 R

n = number of moles = 3 mol

T1 = 25 + 273 = 298 K

T2 = 125 + 273 = 398 K

R = 8.314 J K-1 mol-1

P1 = 1 atm

P2 = 5 atm

Thus,

ΔS = 3 mol x 5/2 R ln 398/298 - 3 mol x R ln 5/1

ΔS = (3 mol x 2.5 x 8.314 J K-1 mol-1 x 0.29) - ( 3 mol x 8.314 J K-1 mol-1 x 1.61)

ΔS = ( 18.08 J K-1 -  40.16 J K-1)

ΔS = - 22.08 J K-1