Question 7.27, Gas constant temperature units

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Sylvia Li
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Joined: Fri Sep 26, 2014 2:02 pm

Question 7.27, Gas constant temperature units

Postby Sylvia Li » Thu Jan 08, 2015 4:39 pm

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.

Terri Shih 3D
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Joined: Fri Sep 26, 2014 2:02 pm

Re: Question 7.27, Gas constant temperature units

Postby Terri Shih 3D » Thu Jan 08, 2015 5:18 pm

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?

Niharika Reddy 1D
Posts: 127
Joined: Fri Sep 26, 2014 2:02 pm

Re: Question 7.27, Gas constant temperature units

Postby Niharika Reddy 1D » Thu Jan 08, 2015 6:05 pm

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.

Sylvia Li
Posts: 10
Joined: Fri Sep 26, 2014 2:02 pm

Re: Question 7.27, Gas constant temperature units

Postby Sylvia Li » Thu Jan 08, 2015 9:03 pm

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!

404324273
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Joined: Tue Nov 25, 2014 3:00 am

Re: Question 7.27, Gas constant temperature units

Postby 404324273 » Tue Jan 13, 2015 12:14 am

I know i'm really late to this post.. but could someone tell me where I get the constant R from?

Neil DSilva 1L
Posts: 70
Joined: Fri Sep 26, 2014 2:02 pm

Re: Question 7.27, Gas constant temperature units

Postby Neil DSilva 1L » Tue Jan 13, 2015 1:37 am

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.

404324273
Posts: 4
Joined: Tue Nov 25, 2014 3:00 am

Re: Question 7.27, Gas constant temperature units

Postby 404324273 » Tue Jan 13, 2015 4:12 pm

Thank You!!

Justin Le 2I
Posts: 142
Joined: Fri Sep 26, 2014 2:02 pm

Re: Question 7.27, Gas constant temperature units

Postby Justin Le 2I » Thu Jan 22, 2015 8:01 pm

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.

Sydniespore
Posts: 14
Joined: Fri Jan 08, 2016 3:00 am

Re: Question 7.27, Gas constant temperature units

Postby Sydniespore » Tue Feb 07, 2017 9:17 pm

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?

Amy_Shao_2D
Posts: 22
Joined: Fri Jul 15, 2016 3:00 am

Re: Question 7.27, Gas constant temperature units

Postby Amy_Shao_2D » Fri Feb 10, 2017 11:15 pm

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.


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