# Chemistry Help!

Discussion in 'Community Discussion' started by TSE, Feb 18, 2009.

1. ### TSE macrumors 68030

Joined:
Jun 25, 2007
Location:
St. Paul, Minnesota
#1
Hey guys, we are studying pressures, temperatures, etc. right now... and on my homework tonight I have a certain question I don't know how to answer...

The gas in a closed container has a pressure of 3.00 multiplied by 102 kPa at 30°C (303 K). What will the pressure be if the temperature is lowered to -172°C?

How do I do this problem and what is the answer? Thanks ahead of time!

2. ### mknawabi macrumors 6502

Joined:
Feb 5, 2006
Location:
Irvine, CA
#2
PV = nRT
(pressure)*(volume) = (#moles)(universal gas constant)(temperature in kelvin)

plug in for pressure. don't count volume since it is a constant anyway. convert -172 to kelvin... do your algebra

3. ### dukebound85 macrumors P6

Joined:
Jul 17, 2005
Location:
5045 feet above sea level
#3
make sure you are using the right units for pressure. depending on how you have the mol ratio

4. ### SLC Flyfishing Suspended

Joined:
Nov 19, 2007
Location:
Portland, OR
#4
You need to specify what the 3 in your pressure means. I'm assuming 3 atm. but if I'm not sure then I can't really help.

Knowing what unit you'll indicate the pressure with is of utmost importance when picking a value for the ideal gas constant.

SLC

5. ### CorvusCamenarum macrumors 65816

Joined:
Dec 16, 2004
Location:
Birmingham, AL
#5
I haven't had chemistry since my undergraduate days, but wouldn't the combined gas law be more effective here?

P1V1/T1 = P2V2/T2
As volume is constant, strike it from the equation: P1/T1 = P2/T2
P1T2 = P2T1
P2 = P1T2/T1
P2 = (P1*101)/303

(I'm a tad confused on the original pressure as well)

?

6. ### mknawabi macrumors 6502

Joined:
Feb 5, 2006
Location:
Irvine, CA
#6
Actually, you are most definitely right.

7. ### SLC Flyfishing Suspended

Joined:
Nov 19, 2007
Location:
Portland, OR
#7
That would be more efficient I suppose!

It's probably bad that I only remember the combined gas law when these sorts of things come up. Not that it's not able to deliver the correct solution, but still.........

SLC

8. ### eddietr macrumors 6502a

Joined:
Oct 29, 2006
Location:
Virginia
#8
The original pressure is 3.00 times 102 kPa. In other words, 306 kPa.

9. ### CorvusCamenarum macrumors 65816

Joined:
Dec 16, 2004
Location:
Birmingham, AL
#9
Assuming you're correct, solving for P2 gives us 102 kPa.

10. ### eddietr macrumors 6502a

Joined:
Oct 29, 2006
Location:
Virginia
#10
Sounds about right to me. And the equation you posted will hopefully help the OP see how you got that.

Intuitively, if :
1. You hold n,R, and V constant (closed container)
2. PV=nRT
3. you reduce the T to about 1/3 of the original condition.

then the pressure must be about 1/3 of the original also.

11. ### TSE thread starter macrumors 68030

Joined:
Jun 25, 2007
Location:
St. Paul, Minnesota
#11
Thanks guys, I will show the professor tomorrow. Thank you for your help!