A gas sample containing 3.00 moles of Helium gas undergoes a state change from 30 degrees celsius and 25.0 L to 45 degrees celsius and 15.0 L. What is the entropy change for the gas (ideal gas)? For He, Cp = 20.8 J/k*mol
ΔS = Cv*ln(Tf/Ti) + nR*ln(Vf/Vi) = Cp*ln(Tf/Ti - nRln(Pf/Pi)
I don't know which equation to use ... I'm given Cp so do I have to convert it to Cv?
Cp = Cv + nR
The Attempt at a Solution
I tried converting Cp to Cv:
Cv = (20.8J/K*mol)(3mol) - (3mol)(8.314J/K*mol)
Cv = 37.458 J/K (not J/Kmol anymore)
ΔS = Cv*ln(Tf/Ti) + nR*ln(Vf/Vi)
ΔS = (37.458)*ln(45/30) + (3)(8.314)*ln(15/25)
ΔS = 2.447 J/k
But the answer is -10.9 J/K.
What am I doing wrong? Thanks