[milani]

Hi, can someone please help me with this question?

An atmospheric chemist, studying the pollutant sulfur trioxide (SO3), places a mixture of sulfur dioxide (SO2) and oxygen (O2) in a rigid 4.10 L container at 941 K and 1.65 atm. When the reaction occurs, gaseous sulfur trioxide is formed according to the balanced equation below and the pressure falls to 1.50 atm.

 

SO2 + ½O2 → SO3

 

Assuming that the temperature at the end of the reaction returns to its original value, what is the partial pressure of sulfur trioxide (SO3) after the reaction?

[sjb]

Where have you got stuck? Why did the pressure drop?

[milani]

i am stuck on how to go about solving the question, my teacher didn't really teach us this.

[Borek]

PV=nRT is all you need.

How many moles of all gases initially? How many moles off all gases after the reaction?

How does the change relate to the reaction stoichiometry?

[milani]

PV=nRT is all you need.

How many moles of all gases initially? How many moles off all gases after the reaction?

How does the change relate to the reaction stoichiometry?

Okay, so I find n before the reaction and then after the reaction... so I have two n values now. How do I go about finding the partial pressure? thanks

[Borek]

You don't need partial pressures.

This is a simple stoichiometry: if one mole of SO₂ reacts, by how many moles does the number of moles of the mixture change?

[milani]

Sorry, i am still a bit confused. Could you please explain the steps to get to the final answer?

[Borek]

Answer the question I asked.

[milani]

if one mole of SO2 reacts, by how many moles does the number of moles of the mixture change?
1?

I have found the initial moles of the reaction and the moles of SO3 in the end as well.

[Borek]

1?

This is not something to guess, you can calculate from the reaction equation.

Assume you have initially - say - 1 mole of SO₂ and 5 moles of O₂. There is an excess of oxygen. How many moles of both gases together?

Now reaction proceeds to the end. 1 mole of SO₂ reacted, there is none left. How many moles of SO₃ were produced? How many moles of O₂ left? How many moles of both gases together?

What is the difference?

Does this difference depend on the number of moles of SO₂ that reacted? If you are not sure - do identical calculations for an initial mixture of 0.5 moles SO₂ and 5 moles of oxygen. For 2 moles SO₂ in 5 moles of oxygen. Actually any number of moles of SO₂ in excess number of moles of oxygen. Do you see a pattern here?

If so, knowing how many moles of the mixture were present the reaction and after the reaction, can you calculate how many moles of SO₂ were present initially in the mixture?