November 14, 2019, 12:53:46 PM
Forum Rules: Read This Before Posting


Topic: How do I begin solving this problem? (about calculating pressure atequilibrium)  (Read 10893 times)

0 Members and 1 Guest are viewing this topic.

Offline o1ocups

  • Regular Member
  • ***
  • Posts: 97
  • Mole Snacks: +0/-0
Consider the following reaction:
2SO2(g) + O2(g) <-> 2SO3(g)
Kp=0.355 at 950 K
A 2.75 L reaction vessel at 950 K initially contains 0.100 mol of SO2 and 0.100 mol of O2.
Calculate the total pressure (in atmospheres) in the reaction vessel when equilibrium is reached.

So, I think I am going to make an ice table:
SO2O2SO3
Initial Concentration0.036 M0.036 M?
Change in Concentration
Equilibrium Concentration

I got the initial concentrations of SO2 and O2 by dividing the number of mols by 2.75 L
But how do you know the initial concentration of SO3?

I used this equation: Kp=Kc/[(RT)^delta n] to find Kc and found it to be 0.00454
Where do I go from here? I am mainly stuck on the initial concentration of SO3.
Or do I even need to find that? I don't know what I should do.

Thanks!!!

Offline Astrokel

  • Full Member
  • ****
  • Posts: 989
  • Mole Snacks: +65/-10
  • Gender: Male
Wasn't the Kp already given? Could you write down Kp expression in term of mole fractions of the reactants and products with the total pressure, P. You have to assume that there is no product initially and that this took place under constant volume.
No matters what results are waiting for us, it's nothing but the DESTINY!!!!!!!!!!!!

Offline Borek

  • Mr. pH
  • Administrator
  • Deity Member
  • *
  • Posts: 25310
  • Mole Snacks: +1659/-398
  • Gender: Male
  • I am known to be occasionally wrong.
    • Chembuddy
But how do you know the initial concentration of SO3?

What about 0?
ChemBuddy chemical calculators - stoichiometry, pH, concentration, buffer preparation, titrations.info, pH-meter.info

Offline o1ocups

  • Regular Member
  • ***
  • Posts: 97
  • Mole Snacks: +0/-0
Wasn't the Kp already given? Could you write down Kp expression in term of mole fractions of the reactants and products with the total pressure, P. You have to assume that there is no product initially and that this took place under constant volume.

Thank you!! I totally forgot that partial pressures and mole fractions are related. Let me try and see if I can get the right answers.

Offline o1ocups

  • Regular Member
  • ***
  • Posts: 97
  • Mole Snacks: +0/-0
But how do you know the initial concentration of SO3?

What about 0?

But the question doesn't say that there isn't any product at the beginning of the reaction?  ???

Offline Astrokel

  • Full Member
  • ****
  • Posts: 989
  • Mole Snacks: +65/-10
  • Gender: Male
That is the assumption you have to make to proceed the question.. Also note it must take place under constant volume.
No matters what results are waiting for us, it's nothing but the DESTINY!!!!!!!!!!!!

Offline o1ocups

  • Regular Member
  • ***
  • Posts: 97
  • Mole Snacks: +0/-0
So...

SO2O2SO3
Initial Concentration0.036 M0.036 M0 M
Change in Concentration-x-0.5x+x
Equilibrium Concentration0.036-x0.036-0.5xx

kc=[SO3]^2/[SO2]^2[O2]
0.00454=x^2/(0.036-x)^2*(0.036-0.5x)
Then, when I tried to solve for x, it just got really complicated
I ended up with -0.00272x^3 - x^2 - 1.47*10^-5x +2.12*10^-7 = 0
I entered the equation into the calculator and I am pretty sure that I got it wrong again.
Is there an easier way to do it?  >:(

Offline Astrokel

  • Full Member
  • ****
  • Posts: 989
  • Mole Snacks: +65/-10
  • Gender: Male
nono, don't work with concentrations.

Ok first calculate the initial partial pressures of SO2 and O2. Then put this into your ice table and work it out with the Kp value given.
No matters what results are waiting for us, it's nothing but the DESTINY!!!!!!!!!!!!

Offline o1ocups

  • Regular Member
  • ***
  • Posts: 97
  • Mole Snacks: +0/-0
nono, don't work with concentrations.

Ok first calculate the initial partial pressures of SO2 and O2. Then put this into your ice table and work it out with the Kp value given.

Yeah...I forgot what I was looking for!! And no I haven't got the pressures yet. Should I use ideal gas law??
And sorry, I have to leave now, but I am going to come back later and work on this. Thank you so much for your help so far. :D

Offline Astrokel

  • Full Member
  • ****
  • Posts: 989
  • Mole Snacks: +65/-10
  • Gender: Male
Quote
Should I use ideal gas law??
Right!

Check this out on the part 'Determining equilibrium pressures': http://www.chem.purdue.edu/gchelp/howtosolveit/Equilibrium/Equilibrium_Concentrations.htm

See how to you ice table with pressure question.
No matters what results are waiting for us, it's nothing but the DESTINY!!!!!!!!!!!!

Offline o1ocups

  • Regular Member
  • ***
  • Posts: 97
  • Mole Snacks: +0/-0
THANK YOU SO MUCH!!!
I got the answer!!  ;)

Sponsored Links