Chemistry Help - Partial Pressure in Decomp

[YoChem]

So I've been working on this for awhile now and I am just not getting to the right answer.

The exact question is:
When 1.7 mol of NOCl was heated to 225 C in a 353 L steel reaction vessel the NOCl partially decomposed according to the following equation:
2NOCl ---->2NO + Cl2
What are the partial pressure of the gases after the reaction? (Cl2, NO, NOCl2)
What percent of NOCl decomposed?

So off the bat I'm assuming that there is 1.7 mol of NOCl, 1.7 mol NO and .85 mol of Cl2.
After that I've tried a few different things. If someone could explain the steps to me that would be great!

 

[Borek]

There is not enough information to solve the question. You are told NOCl decomposed PARTIALLY - so amount of gases produced is not known.

Is this whole question? What part of the course (and what course) are you doing now?

 

[YoChem]

Sorry about that. I did leave a part out. It says the final pressure after the reaction is .26 atm. Its from the Mastering Chemistry software and its for Chem 1 and we are currently studying partial pressure.
We have not gone over ICE charts or anything overtly compicated. Any thoughts?

 

[wajagawa]

I am not too sure if I follow you based on the information you provided. But, (and somebody please correct me if I am wrong) you need to find out the amount moles of each substance present at the end of the reaction. From that you would then need to determine the mole fraction of each substance at the end of the reaction. Assuming the gasses act ideally (so that we can simplify the problem), each substance present at the end of reaction contributes to the final pressure (0.26 atm) in proportion to its mole fraction (e.g. P_NO= X_NO*P_final).

However, as Borek explained before there is insufficient information to obtain the answer.

 

[Borek]

Together with pressure it can be solved. Reaction equation gives you information about ratio of number of moles of gases present. Total pressure is sum of partial pressures, you know ratio of partial pressures - that gives you two equations in two unknowns, simple algebra to solve.

--

 

[YoChem]

So what would be the total amount of moles of gas? is it 5 (2+2+1) or 4.25 (1.7+1.7+.85) and would the pressure it gets multiplied by be .26 (the final pressure in the vessel) or is it .2 (P=((1.7)(.08206)(498)/353)).
Sorry I am lost with this, my instructor kind of breezed though this information, so I'm trying to mostly teach myself.

 

[Borek]

Total AMOUNT of gas doesn't matter.

You have two unknown partial pressures - P_NO and P_Cl2. What is their sum? Can you tell - looking at the reaction stoichiometry - what is ratio P_NO/P_Cl2?

 

[YoChem]

Sorry maybe I am not seeing this clearly.
Dont you need to know the total amount of moles to figure out the partial pressure?
(mol of molecule/total mol)*(pressure)?
So in this case the moles of NOCl is 1.7. The mol of NO is the same 1.7 and the mol of Cl2 is half that .85. Then I would divide each by the total amount of mol and multiply each by .26?
Im I on the right track with this?

 

[Borek]

Im I on the right track with this?

No. Try to answer my questions.

 

[YoChem]

In your question your'e saying I have two unknown partial pressure, but don't i have 3? The NOCl as well.
The sum of all the partial pressures though have to equal .26 atm. The ratio of NO/Cl2 is 1:2 correct?

 

[YoChem]

Well I got the correct answer.
If someone wanted to check my work just to make sure it wasnt blind luck.
1) I wanted to know how many moles are supposed to be in the final reaction so through the ideal gas law the final reaction should have 2.25 moles.
2) So we started with 1.7 moles of NOCl to get the percentage of the reaction that was decomposed I subtracted 2.24-1.7 then divided that by 1.7 to get 33 percent DID not compose.
3. 33% of 1.7 gives you moles of NOCl left then subtracting that from 2.25 I was able to find the moles of the other compounds. By using the mol ratio I was able to find the exact moles of those compounds and then was able to get the partial pressure.
What do you think?

 

[Borek]

Sorry, I am an idiot. After seeing the question and seeing that it can't be solved later for some reason I was all the time sure you started with solid, not with a gas - and after making my mind initially I have not bothered to look again at the problem. My mistake. But...

The sum of all the partial pressures though have to equal .26 atm. The ratio of NO/Cl2 is 1:2 correct?

this is still the right starting point.

And 2.25 moles of total gas is an important information. 2.25-1.7=0.55 is a difference in number of moles between initial mixture and the final mixture. 2 moles of NOCl give 2 moles on NO, so there is no change in the number of moles here. Can you say 0.55 moles of what appeared? Can you use stoichiometry to calculate number of moles of NOCl that decomposed?

33% doesn't look OK to me, correct result is almost twice that.

More systematic approach would be to first calculate number of moles change per mole of decomposed NOCl, than - knowing that change in number of moles was 0.55 - use ratio to calculate number of moles NOCl that decomposed.