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### Topic: How do I begin solving this problem? (about calculating pressure atequilibrium)  (Read 11075 times)

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#### o1ocups

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##### How do I begin solving this problem? (about calculating pressure atequilibrium)
« 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:
 SO2 O2 SO3 Initial Concentration 0.036 M 0.036 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!!!

#### Astrokel

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##### Re: How do I begin solving this problem? (about calculating pressure atequilibrium)
« Reply #1 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.
No matters what results are waiting for us, it's nothing but the DESTINY!!!!!!!!!!!!

#### Borek

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##### Re: How do I begin solving this problem? (about calculating pressure atequilibrium)
« Reply #2 on: February 15, 2009, 05:34:34 PM »
But how do you know the initial concentration of SO3?

ChemBuddy chemical calculators - stoichiometry, pH, concentration, buffer preparation, titrations.info, pH-meter.info

#### o1ocups

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##### Re: How do I begin solving this problem? (about calculating pressure atequilibrium)
« Reply #3 on: February 15, 2009, 05:36:32 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.

#### o1ocups

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##### Re: How do I begin solving this problem? (about calculating pressure atequilibrium)
« Reply #4 on: February 15, 2009, 05:42:20 PM »
But how do you know the initial concentration of SO3?

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

#### Astrokel

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##### Re: How do I begin solving this problem? (about calculating pressure atequilibrium)
« Reply #5 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.
No matters what results are waiting for us, it's nothing but the DESTINY!!!!!!!!!!!!

#### o1ocups

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##### Re: How do I begin solving this problem? (about calculating pressure atequilibrium)
« Reply #6 on: February 15, 2009, 06:10:29 PM »
So...

 SO2 O2 SO3 Initial Concentration 0.036 M 0.036 M 0 M Change in Concentration -x -0.5x +x Equilibrium Concentration 0.036-x 0.036-0.5x x

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?

#### Astrokel

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##### Re: How do I begin solving this problem? (about calculating pressure atequilibrium)
« Reply #7 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.
No matters what results are waiting for us, it's nothing but the DESTINY!!!!!!!!!!!!

#### o1ocups

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##### Re: How do I begin solving this problem? (about calculating pressure atequilibrium)
« Reply #8 on: February 15, 2009, 06:30:10 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.

#### Astrokel

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##### Re: How do I begin solving this problem? (about calculating pressure atequilibrium)
« Reply #9 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.
No matters what results are waiting for us, it's nothing but the DESTINY!!!!!!!!!!!!

#### o1ocups

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##### Re: How do I begin solving this problem? (about calculating pressure atequilibrium)
« Reply #10 on: February 16, 2009, 01:28:08 AM »
THANK YOU SO MUCH!!!