Using gas law to find moles of SO3 formed

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Discussion Overview

The discussion revolves around a chemistry homework problem involving the reaction of sulfur dioxide (SO2) and oxygen (O2) to form sulfur trioxide (SO3). Participants explore the application of the ideal gas law and stoichiometry to determine the number of moles of SO3 produced in a closed system under specific conditions of pressure and volume.

Discussion Character

  • Homework-related
  • Mathematical reasoning
  • Technical explanation
  • Exploratory

Main Points Raised

  • One participant calculates the initial number of moles of gas using the ideal gas law but expresses uncertainty about how to proceed from that point.
  • Another participant suggests using the concept of chemical equilibrium, though this suggestion is met with confusion.
  • A participant proposes that the total number of moles of gas remains constant, leading to a method for calculating the final number of moles based on the final pressure.
  • There is a discussion about the conservation of atoms versus the conservation of molecules, with a clarification that the total number of atoms is conserved in the reaction.
  • A participant calculates the final number of moles of gas but questions the interpretation of the balanced chemical equation in terms of moles produced and consumed.
  • Another participant confirms that not all produced gas is SO3 and emphasizes the need to consider the stoichiometry of the reaction, noting the net loss of moles in the process.

Areas of Agreement / Disagreement

Participants express differing views on the assumptions regarding the conservation of moles and the interpretation of the balanced equation. There is no consensus on the exact approach to take or the assumptions that can be made regarding the reaction conditions.

Contextual Notes

Participants highlight the importance of maintaining constant volume and temperature in the calculations, but there are unresolved questions about the exact amounts of reactants and products involved in the reaction.

physgirl
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Homework Statement


A chemist studying the rxns of pollutant SO2 places a mixture of SO2 and O2 in a 2.0L container at 900K and an P[init]=1.95 atm. When rxn occurs, gaseous SO3 forms and P falls to 1.65 atm. How many moles of SO3 formed?

Homework Equations


PV=nRT but I'm not sure what else...

The Attempt at a Solution


I was assuming both SO2 and O2 in the starting material are gases? So I used V=2L, P=1.95atm, and T=900K to find the number of moles of both gases combined, which turned out to be ~0.053 moles. And I'm not sure what to do with this number to get to the answer... I wrote out the balanced equation to be: 2SO2(g) + O2(g) --> 2SO3 (g) although I don't know if that matters... I can't assume that T stays the same, can I? the problem doesn't say anything about it...

[edit] wait, can I start out by saying that... since you're not losing any gas, #moles of starting material (SO2+O2) has to be the same as #moles of final mixture (SO3+(leftover)SO2+O2)... and so use 0.053moles, given final pressure (1.65atm) and 2L (since it's a container it doesn't expand..?) to find the final temperature and go from there somehow?
 
Last edited:
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Try something called Chemical Equilibrium.
 
hmm...? :-/
 
physgirl said:
[edit] wait, can I start out by saying that... since you're not losing any gas, #moles of starting material (SO2+O2) has to be the same as #moles of final mixture (SO3+(leftover)SO2+O2)... and so use 0.053moles, given final pressure (1.65atm) and 2L (since it's a container it doesn't expand..?) to find the final temperature and go from there somehow?
No, you have to assume that the volume of the container and the temperature are unchanged. And the number of moles is different from the number of atoms. It is the total number of atoms that is conserved, not the total number of molecules (or moles).

1. Find the final number of moles using the final pressure.
2. Translate the balanced equation into words.
 
Allright, don't try something called Chemical Equilibrium.
Try something called Follow What Gokul Pointed Out.
:biggrin:
 
so I did P(i)/n(i)=P(f)/n(f)... and got the final #moles to be 0.0447moles of gas... but I'm not sure what you mean by "translate the balanced equation into words."... so from 3 moles of gas you make 2 moles of SO3 gas...?

Im guessing you can't assume that not all of 0.0447 moles of what's produced is SO3 because you can't assume that you had the exact amount of starting material needed to produce that, so the product gas mixture is a combination of all 3 gases?
 
Correct. All that's produced is NOT SO3.

so from 3 moles of gas you make 2 moles of SO3 gas
This is close. You need to expand on this idea. So, for every 3 moles consumed, there are 2 moles produced - resulting in a net loss of 1 mole.

Thus, 1 mole net loss => 2 moles SO2 produced.
So (0.053-0.0447) moles net loss => ?? moles of SO2 produced.
 
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