Calculating Entropy Change in an expansion

[iraya92]

1. Suppose that you have a sample of a gas in a cylinder equipped with a piston that has a volume of 1.50 L, a pressure of 1.20 atm, and a temperature of 250 K. Suppose that the gas is expanded reversibility under isothermal conditions until the pressure is 0.75 atm. What is the entropy change that accompanies this expansion?

Homework Equations

dS= nRln (V2/V1) [/B]

The Attempt at a Solution

So I calculated V2 using p1V1=p2V2 and got 2.4L. I plugged that into the equation above but got 3.9...The answer is suppossed to be 0.34. Can you guys help me? :) [/B]

 

[Chestermiller]

1. Suppose that you have a sample of a gas in a cylinder equipped with a piston that has a volume of 1.50 L, a pressure of 1.20 atm, and a temperature of 250 K. Suppose that the gas is expanded reversibility under isothermal conditions until the pressure is 0.75 atm. What is the entropy change that accompanies this expansion?

Homework Equations

dS= nRln (V2/V1) [/B]

The Attempt at a Solution

So I calculated V2 using p1V1=p2V2 and got 2.4L. I plugged that into the equation above but got 3.9...The answer is suppossed to be 0.34. Can you guys help me? :) [/B]

What are the units of your answer, and what are the units of the accepted answer? What value did you get for n?

 

[iraya92]

The units are 0.34 J/K and as for the moles I think we were to assume the gas to be one mole.

 

[iraya92]

Oh Okay I got it... so I was to calculate n too...duh...Thanks! Got it... *Feeling enlightened*