Temperature in Entropy Change for Monoatomic Ideal Gas Midterm Question 6B

Volume: $\Delta S = nR\ln \frac{V_{2}}{V_{1}}$
Temperature: $\Delta S = nC\ln \frac{T_{2}}{T_{1}}$

Alexia Joseph 2B
Posts: 56
Joined: Thu Jul 27, 2017 3:01 am

Temperature in Entropy Change for Monoatomic Ideal Gas Midterm Question 6B

Hi,

This question asks to calculate the entropy change for a monoatomic ideal gas at 1 atm, 303 K, and in a balloon of 53.48 L when the gas is heated to 333 K at a constant volume.

When calculating the n=PV/RT, why do you use the initial temperature instead of the final temperature?

lauren chung 2f
Posts: 50
Joined: Fri Sep 29, 2017 7:03 am

Re: Temperature in Entropy Change for Monoatomic Ideal Gas Midterm Question 6B

The temperature and volume is given at a pressure of 1 atm, so you have all of the elements that you need to plug into PV=nRT and find n. When the temperature changes, you don't know if the pressure is constant. So you cannot use the final temperature because you don't know what the final pressure is, but you do know the initial temperature and pressure.

Pooja Nair 1C
Posts: 55
Joined: Thu Jul 13, 2017 3:00 am

Re: Temperature in Entropy Change for Monoatomic Ideal Gas Midterm Question 6B

When you calculate n, you know the information before the temperature change. Since the rest of the variables (in a more general example) can change during a temperature change, you should calculate n at the state in which the only unknown is n, which in this case, is when T = 303 K