Chemistry Help - Partial Pressure in Decomp

Click For Summary

Discussion Overview

The discussion revolves around calculating the partial pressures of gases produced from the partial decomposition of NOCl in a chemical reaction. Participants explore the application of the ideal gas law and stoichiometry in determining the amounts of gases present after the reaction, as well as the percentage of NOCl that decomposed. The context is a chemistry homework problem related to partial pressure concepts.

Discussion Character

  • Homework-related
  • Mathematical reasoning
  • Debate/contested

Main Points Raised

  • One participant states that with 1.7 mol of NOCl, they assume 1.7 mol of NO and 0.85 mol of Cl2 are produced, but they are unsure of the steps to find the partial pressures.
  • Another participant points out that there is insufficient information to solve the question due to the partial decomposition of NOCl.
  • A later reply clarifies that the final pressure is given as 0.26 atm, and the problem is part of a chemistry course focused on partial pressure.
  • One participant suggests that to find the partial pressures, the mole fractions of each gas must be determined, assuming ideal gas behavior.
  • Another participant mentions that the total pressure is the sum of the partial pressures and that the reaction stoichiometry provides a ratio of moles, leading to two equations with two unknowns.
  • There is confusion about the total amount of gas and whether it should be calculated as 5 or 4.25 moles, with participants discussing how to apply the ideal gas law.
  • One participant asserts that the sum of all partial pressures must equal 0.26 atm and questions the ratio of NO to Cl2.
  • Another participant claims to have arrived at the correct answer by calculating the total moles of gas and the percentage of NOCl that decomposed, but others challenge the correctness of their calculations.
  • One participant admits to a misunderstanding regarding the state of the reactants and emphasizes the importance of stoichiometry in determining the moles of NOCl that decomposed.

Areas of Agreement / Disagreement

Participants express differing views on how to approach the problem, with some asserting that there is not enough information to solve it, while others believe they can derive the necessary values using stoichiometry and the ideal gas law. The discussion remains unresolved, with multiple competing views on the correct methodology and calculations.

Contextual Notes

There are limitations in the information provided, particularly regarding the initial conditions and the specifics of the decomposition reaction. Participants also express uncertainty about the calculations and the assumptions made regarding ideal gas behavior and stoichiometric relationships.

YoChem
Messages
7
Reaction score
0
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!
 
Physics news on Phys.org
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?
 
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?
 
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. PNO= XNO*Pfinal).

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

--
 
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.
 
Total AMOUNT of gas doesn't matter.

You have two unknown partial pressures - PNO and PCl2. What is their sum? Can you tell - looking at the reaction stoichiometry - what is ratio PNO/PCl2?
 
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?
 
YoChem said:
Im I on the right track with this?

No. Try to answer my questions.
 
  • #10
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?
 
  • #11
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?
 
  • #12
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...

YoChem said:
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.
 

Similar threads

How to Calculate Partial Pressure of NOCl in a Chemical Reaction?
  • · Replies 1 ·
Replies
1
Views
2K
Partial Pressure Quest: 2L Vessel, PCl5 Decomposition @ 250C
  • · Replies 6 ·
Replies
6
Views
2K
Ideal Gas Law solve for partial pressure
  • · Replies 2 ·
Replies
2
Views
4K
What Are the Partial Pressures of SO2, Cl2, and SO2Cl2 at Equilibrium?
  • · Replies 1 ·
Replies
1
Views
9K
Chemistry homework help with enthelpy
  • · Replies 3 ·
Replies
3
Views
2K
What are the partial pressures of H2, Br2, and HBr at equilibrium?
  • · Replies 2 ·
Replies
2
Views
10K
Chemistry Partial pressure and moles problem?
  • · Replies 2 ·
Replies
2
Views
5K
Equilibrium of a gas mixture from partial pressures
  • · Replies 4 ·
Replies
4
Views
8K
Partial pressure of reagent in reaction
  • · Replies 1 ·
Replies
1
Views
2K