Partial Pressures: Solving Complex Equations

[Qube]

Homework Statement

http://i.minus.com/jbzyIAyMvUrADW.png

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?

 

[Borek]

Number of moles of what is calculated?

 

[Qube]

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]

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.

 

[Qube]

I don't understand what I posted

 

[Borek]

Do you understand

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

?

 

[Qube]

I understand that but nothing else.

 

[Borek]

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

 

[Qube]

No, there are two moles on either side

 

[Borek]

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?

 

[Qube]

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

 

[Borek]

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

What is a mole definition?

 

[Qube]

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

 

[Borek]

OK, you have a mixture of n_NO2 (containing 1 mole of nitrogen atoms per one mole of gas) and n_N2O4 (containing 2 moles of nitrogen atoms per one mole of gas). How many moles of nitrogen atoms are in the mixture?

 

[Qube]

There are n plus 2n moles of nitrogen.

 

[Qube]

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

 

[Borek]

OK, so now you know that the mixture contains n_NO2+2n_N2O4 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.