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Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: o1ocups on February 15, 2009, 05:03:21 PM

Title: How do I begin solving this problem? (about calculating pressure atequilibrium)
Post by: o1ocups on February 15, 2009, 05:03:21 PM
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!!!
Title: Re: How do I begin solving this problem? (about calculating pressure atequilibrium)
Post by: Astrokel on February 15, 2009, 05:33:21 PM
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.
Title: Re: How do I begin solving this problem? (about calculating pressure atequilibrium)
Post by: Borek on February 15, 2009, 05:34:34 PM
Quote from: o1ocups on February 15, 2009, 05:03:21 PM
But how do you know the initial concentration of SO3?

What about 0?
Title: Re: How do I begin solving this problem? (about calculating pressure atequilibrium)
Post by: o1ocups on February 15, 2009, 05:36:32 PM
Quote from: Astrokel on February 15, 2009, 05:33:21 PM
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.
Title: Re: How do I begin solving this problem? (about calculating pressure atequilibrium)
Post by: o1ocups on February 15, 2009, 05:42:20 PM
Quote from: Borek on February 15, 2009, 05:34:34 PM
Quote from: o1ocups on February 15, 2009, 05:03:21 PM
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?  ???
Title: Re: How do I begin solving this problem? (about calculating pressure atequilibrium)
Post by: Astrokel on February 15, 2009, 05:45:46 PM
That is the assumption you have to make to proceed the question.. Also note it must take place under constant volume.
Title: Re: How do I begin solving this problem? (about calculating pressure atequilibrium)
Post by: o1ocups on February 15, 2009, 06:10:29 PM
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?  >:(
Title: Re: How do I begin solving this problem? (about calculating pressure atequilibrium)
Post by: Astrokel on February 15, 2009, 06:14:42 PM
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.
Title: Re: How do I begin solving this problem? (about calculating pressure atequilibrium)
Post by: o1ocups on February 15, 2009, 06:30:10 PM
Quote from: Astrokel on February 15, 2009, 06:14:42 PM
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
Title: Re: How do I begin solving this problem? (about calculating pressure atequilibrium)
Post by: Astrokel on February 15, 2009, 06:49:41 PM
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.
Title: Re: How do I begin solving this problem? (about calculating pressure atequilibrium)
Post by: o1ocups on February 16, 2009, 01:28:08 AM
THANK YOU SO MUCH!!!
I got the answer!!  ;)