Calculating Entropy Change in an expansion

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Discussion Overview

The discussion revolves around calculating the entropy change during the isothermal expansion of a gas in a cylinder. Participants are addressing a homework problem involving the application of the entropy change formula and the ideal gas law.

Discussion Character

  • Homework-related
  • Mathematical reasoning

Main Points Raised

  • One participant presents a scenario involving a gas expansion and attempts to calculate the entropy change using the formula dS = nRln(V2/V1).
  • The same participant calculates V2 as 2.4 L but initially arrives at an incorrect entropy change value of 3.9 J/K.
  • Another participant questions the units of the answer and the value of n, prompting clarification on the assumptions made regarding the number of moles of gas.
  • A later reply indicates that the participant realizes the need to calculate the number of moles, suggesting a moment of understanding regarding the problem.

Areas of Agreement / Disagreement

The discussion does not reach a consensus on the correct entropy change value, as participants are still working through the calculations and assumptions involved.

Contextual Notes

Participants have not fully resolved the assumptions regarding the number of moles of gas or the calculations leading to the entropy change. There is also a lack of clarity on the relationship between the calculated and accepted values.

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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