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Question

Find out the change in entropy ΔS in J K1 when 4 mol of Helium gas is isobarically heated from 250 K to 350 K.
Assume Cp,m=5.19 kJ mol1 K1

A
4812
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B
3950
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C
6989
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D
7050
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Solution

The correct option is C 6989

Given here,
T1=250 KT2=350 Kn=4 molCp,m=5.19 kJ mol1 K1

Change in entropy for an isobaric process is given by:
ΔS= n×Cp,m×lnT2T1

putting the values,
ΔS=[4×5.19×ln350250] kJ K1ΔS=6.9886 kJ K1ΔS=6988.6 J K1


Theory:

Entropy Change in an Isobaric Process :
we know,
ΔSsys=nCv,mlnT2T1+nRlnV2V1
When pressure is constant;
V2V1=T2T1

Taking common, n lnT2T1
ΔSsys=n lnT2T1(Cv,m+R)
We know,
Cp,mCv,m=R
Cv,m+R=Cp,m

ΔSsys=nCp,m lnT2T1

Also from , ΔSsys=nCp,mlnT2T1+nRlnP1P2
at constant P
ΔSsys=nCp,m lnT2T1

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