Calculating Entropy Change in an expansion

AI Thread Summary
In a discussion about calculating entropy change during the isothermal expansion of a gas, a user presented a scenario involving a gas in a cylinder with specific initial conditions. They initially calculated the final volume using the ideal gas law but arrived at an incorrect entropy change value. After clarifying the need to consider the number of moles, which was assumed to be one, they recalculated and confirmed the correct entropy change as 0.34 J/K. The conversation highlighted the importance of including all variables in thermodynamic calculations. The user expressed satisfaction after resolving their confusion.
iraya92
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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]
 
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iraya92 said:
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?
 
The units are 0.34 J/K and as for the moles I think we were to assume the gas to be one mole.
 
Oh Okay I got it... so I was to calculate n too...duh...Thanks! Got it... *Feeling enlightened*
 
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