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## Homework Statement

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**

## Homework Equations

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)

Then,

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