# Partial Pressures

Gold Member

## Homework Equations

Conservation of mass in a closed system.

## The Attempt at a Solution

I'm not sure what the lecture is getting at here. Why is it that the number of moles of NO2 added to twice the number of moles of N2O4 = the total moles? Shouldn't it be the other way around?

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Borek
Mentor
Number of moles of what is calculated?

Gold Member
I think 0.330 moles refer to the total moles of gas in the flask. I'm not sure why the equivalency on the bottom of the slide holds.

Borek
Mentor
I think 0.330 moles refer to the total moles of gas in the flask.
Have you READ the text you posted? It clearly explains what is calculated.

Gold Member
I don't understand what I posted

Borek
Mentor
Do you understand

0.330 mol of NO2 and therefore the same number of moles of nitrogen atom atoms
?

Gold Member
I understand that but nothing else.

Borek
Mentor
Does the number of moles of N atoms change throughout the reaction?

Gold Member
No, there are two moles on either side

Borek
Mentor
OK (although the reason is slightly different - mass is conserved, nitrogen is not produced out of nothing, actually the fact that there are two moles on both sides on of the reaction equation is a conclusion of the mass conservation, not a proof of the mass conservation as you seem to be implying. Also, it doesn't have to be two moles - what if you start with half o mole of nitrogen atoms?).

There is an identical amount of nitrogen atoms before and after the reaction.

How many moles of nitrogen atoms per mole of NO2?

How many moles of nitrogen atoms per mole of N2O4?

Gold Member
There are n moles of nitrogen atoms per mole of NO2 and 2n moles of nitrogen atoms per mole of N2O4.

Borek
Mentor
No, not "n moles" per mole. n has a well known value.

What is a mole definition?

Gold Member
There is one mole of N in one mole of nitrogen dioxide. And two moles of N in one mole of dinitrogen tetraoxide

Borek
Mentor
OK, you have a mixture of nNO2 (containing 1 mole of nitrogen atoms per one mole of gas) and nN2O4 (containing 2 moles of nitrogen atoms per one mole of gas). How many moles of nitrogen atoms are in the mixture?

Gold Member
There are n plus 2n moles of nitrogen.

Gold Member
More specifically, the first n refers to nitrogen dioxide and the second n refers to dinitrogen tetraoxide.

Borek
Mentor
OK, so now you know that the mixture contains nNO2+2nN2O4 moles of nitrogen atoms, you know that initially there were 0.330 moles of nitrogen atoms, and you know that amount of nitrogen atoms didn't change. Get back to the original text.