# Chemical Forums

## Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: o1ocups on February 16, 2009, 02:32:39 AM

Title: Another problem about predicting the pressure-am I on the right track?
Post by: o1ocups on February 16, 2009, 02:32:39 AM
Question

At a given temperature a system containing  and some oxides of nitrogen can be described by the following reactions:
2NO(g) + O2(g) ::equil:: 2No2(g) Kp=104
2NO2(g) ::equil:: N2O4(g) Kp=0.10
A pressure of 1 atm of N2O4(g) is placed in a container at this temperature.

Predict which, if any component (other than ) will be present at a pressure greater than 0.2 atm at equilibrium.
A) The pressures of NO and O2 will be negligible.
B) NO will be present.
C) NO and O2 will be present.
D) O2 will be present.

What I attempted

2NO(g) + O2(g) ::equil:: 2NO2(g) Kp=104
2NO2(g) ::equil:: N2O4(g) Kp=0.10

2NO(g) + O2(g) ::equil:: N2O4(g)
104*0.10=10000=Kp

Kp=PN2O4/PO2PNO

PO2PNO=Kp/PN2O4

PO2PNO=1/10000=negligible??

I am not sure what I am doing. Is this right?  ???
Title: Re: Another problem about predicting the pressure-am I on the right track?
Post by: Astrokel on February 16, 2009, 10:57:38 AM
Quote
2NO(g) + O2(g)  N2O4(g)
104*0.10=10000=Kp
ICE table. What do you see after putting it into your Kp expression?
Title: Re: Another problem about predicting the pressure-am I on the right track?
Post by: o1ocups on February 16, 2009, 01:58:33 PM
 NO O2 N2O4 I ? ? 1 atm C E 0.2?? 0.2??

I don't understand what you mean  :-[
Sorry, I am really slow.
Title: Re: Another problem about predicting the pressure-am I on the right track?
Post by: Astrokel on February 16, 2009, 05:17:21 PM
It's ok, take it slowly as long as you give it a try. Always follow the question and it says you only have 1 atm of N2O4, so assumption is NO and O2 is present at 0 initially. Ok so start working with dissociation next.

2NO  O2  ::equil:: N2O4
I    0    0          1 atm
C  +2x  +x        -x
E

i guess you know what to do next and just observe what you get.
Title: Re: Another problem about predicting the pressure-am I on the right track?
Post by: o1ocups on February 16, 2009, 06:46:14 PM
Thank you SO MUCH for your time and energy!! I got the right answer!  :D
I just find the information about the initial conditions really ambiguous. I never know what to assume  :-\
But thanks again!!
Title: Re: Another problem about predicting the pressure-am I on the right track?
Post by: pandabear on February 16, 2009, 07:49:42 PM
i still don't understand what the answer is...sorry, i'm an idiot.
Title: Re: Another problem about predicting the pressure-am I on the right track?
Post by: o1ocups on February 16, 2009, 08:31:48 PM
i still don't understand what the answer is...sorry, i'm an idiot.

I'll give you a hint: how do you calculate Kp?