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Topic: Concentration of nitrogen oxide in air  (Read 2052 times)

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Offline Woopy

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Concentration of nitrogen oxide in air
« on: October 13, 2013, 06:28:54 PM »
The relative compositions of the pollutants NO and NO2 in air are governed by the reaction

NO + 1/2 O2 ⇔NO2

For air containing 21-mol% O2 at 25°C and 1.0133 bar, what is the concentration of NO in parts per million if the total concentration of both nitrogen oxides is 5 ppm? Note ppm is taken on a molar basis for gases.




For this problem I am not sure how to go about it. I am not 100% sure if ppm means moles/total moles. Also, I am not able to find out the concentration of NO2 at equilibrium, which is why I haven't been able to get the concentration of NO.
« Last Edit: October 14, 2013, 04:12:48 PM by billnotgatez »

Offline Borek

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Re: Concentration of nitrogen oxide in air
« Reply #1 on: October 14, 2013, 03:33:27 AM »
ppm means either volume/volume (so moles over moles is OK) or weight/weight (so moles/moles would be wrong). As ppm is ambiguous, it should be IMHO clearly stated in the problem.

I don't see how to solve the problem without knowing equilibrium constant.
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Offline Enthalpy

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Re: Concentration of nitrogen oxide in air
« Reply #2 on: October 14, 2013, 09:27:01 PM »
[...] Note ppm is taken on a molar basis for gases. [...]

And I don't see clearly how to compute an "equilibrium" between 5ppm NO+NO2 in air at 25°C 1atm, while measured concentration in urban area is like 0.01ppm, suggesting that the equilibrium concentration is far less.

In other words: the given reaction determines nothing. Both NO and NO2 concentrations result from the production and destruction speeds far from equilibrium, and if some sub-ppb equilibrium was reached, then the governing reaction would be with N2 and O2...

But, well, never risk an answer more clever than the question.

So to provide the expected answer, get the equilibrium constant (or entropy of formation, or Gibbs, or µ) versus the elements, deduce it for the NO versus NO2 reaction, and paste the false value where desired to be a good student.

Offline Woopy

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Re: Concentration of nitrogen oxide in air
« Reply #3 on: October 14, 2013, 11:50:04 PM »
This chart was in the lecture slide, and is the only thing I could use to get maybe a K value, it looks like I need to extrapolate from the data. The value is ridiculously low. I mean, out of 5 ppm, I have a ppm per ppm on my NO concentration, practically zero

Offline Woopy

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Re: Concentration of nitrogen oxide in air
« Reply #4 on: October 15, 2013, 09:35:28 PM »
I also realized you can find K with the equation k = exp(-ΔG/RT)

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