## Question 7.27, Gas constant temperature units

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

### Question 7.27, Gas constant temperature units

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
Posts: 6
Joined: Fri Sep 26, 2014 2:02 pm

### Re: Question 7.27, Gas constant temperature units

Celsius and Kelvin go by the same increments though, so the change in temperature should be the same regardless of your unit.

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

### Re: Question 7.27, Gas constant temperature units

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

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

### Re: Question 7.27, Gas constant temperature units

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

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

Thank You!!

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

### Re: Question 7.27, Gas constant temperature units

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

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

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.

Return to “Reaction Enthalpies (e.g., Using Hess’s Law, Bond Enthalpies, Standard Enthalpies of Formation)”

### Who is online

Users browsing this forum: No registered users and 1 guest