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Question 7.27, Gas constant temperature units

Posted: Thu Jan 08, 2015 4:39 pm
by Sylvia Li
The question asks:
Calculate the heat released by 5.025 g of Kr(g) at 0.400 atm as it cools from 97.6 degrees C to 25.0 degrees C at
(a) constant pressure and
(b) constant volume.
Assume that krypton behaves as an ideal gas.

I'm using the equations q=n*delta T*C(p,m) and C(p,m)=(5/2)*R.
The only part I'm having trouble with is the temperature units to use for change in temperature and the gas constant R.
In the solutions manual, R is 8.314 (it gives C(p,m) as 20.8 J*mol^-1*C^-1), but isn't 8.314 normally in terms of Kelvin?

I tried to convert the change in temperature to Kelvin and, of course, my answers were way off.

Re: Question 7.27, Gas constant temperature units

Posted: Thu Jan 08, 2015 5:18 pm
by Terri Shih 3D
Celsius and Kelvin go by the same increments though, so the change in temperature should be the same regardless of your unit.
Could you post your calculations?

Re: Question 7.27, Gas constant temperature units

Posted: Thu Jan 08, 2015 6:05 pm
by Niharika Reddy 1D
When converting to Kelvin, convert the initial and final temperatures first, and then find the change in temperature from the two temperatures in Kelvin. I'm not sure if this is what you did, but if you find delta T in °C first and then convert that to Kelvin, it won't give you the correct answer.

Re: Question 7.27, Gas constant temperature units

Posted: Thu Jan 08, 2015 9:03 pm
by Sylvia Li
Oh, I see! Yes, I converted the change in temperature to Kelvin, not the initial and final values by themselves. I have the correct answer now. Thanks for your help!

Re: Question 7.27, Gas constant temperature units

Posted: Tue Jan 13, 2015 12:14 am
by 404324273
I know i'm really late to this post.. but could someone tell me where I get the constant R from?

Re: Question 7.27, Gas constant temperature units

Posted: Tue Jan 13, 2015 1:37 am
by Neil DSilva 1L
R is the gas constant. It can be found online, on the back of the periodic table that came with the course reader, and in your textbook.

Re: Question 7.27, Gas constant temperature units

Posted: Tue Jan 13, 2015 4:12 pm
by 404324273
Thank You!!

Re: Question 7.27, Gas constant temperature units

Posted: Thu Jan 22, 2015 8:01 pm
by Justin Le 2I
Also, be careful about which R you pick because they have different units. Look at the units in your calculations and pick the one that fits.

Re: Question 7.27, Gas constant temperature units

Posted: Tue Feb 07, 2017 9:17 pm
by Sydniespore
Why do they round up in the beginning rather than the end. I thought we only do sig figs at the end of a calculation?

Re: Question 7.27, Gas constant temperature units

Posted: Fri Feb 10, 2017 11:15 pm
by Amy_Shao_2D
Yes, you usually save doing sig figs till the very end. Sometimes solutions will round ahead of time because they probably don't want to have to write out the entire number, but your end answer should be the same.