Chemical Forums
Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: anubistsai on August 09, 2007, 05:28:00 PM
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Well, ran into this problem today:
Pure phosgene gas (COCl2), 3.00e-2 mol, was placed in a 1.50 L container. It was heated to 800K and at equilibrium the pressure of CO was found to be 0.497 atm. calculate the equilibrium constant Kp for the reaction:
CO(g)+Cl2(g)=COCL2(g)
The answer given is 3.3
I only figures that I probably should use partial pressure to solve this problem, but I don't know it's supposed to be the partial pressure of what.
Thanks.
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pCO V = nCO R T; this will give you nCO (mind the units; use m3 and Pa if you use the common gas constant of 8.3145 J / mole K)
pCO = partial pressure of CO
furthermore, you know 3e-2 moles of PURE COCl2 were put in; so:
nstart COCl2 = 3e-2
nCO = nCl (for every CO formed, there is also a Cl2 formed) = calculated above
nend COCl2 = nstart COCl2 - nCO (every mole CO formed means 1 mole COCl2 less
this will give you the molar numbers, and thus the total amount of moles.
ntotal = nCO + nCl + nend COCl2
you now have the number of moles, so can calculate the total Pressure of the mixture.
partial pressures are of course mole fraction * total Pressure
and:
Kp = partial pressure of COCl2 / partial pressure of CO * partial pressure of Cl2
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Answer still doesn't make sense. :P What is this total pressure that you were referring to?
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total pressure is the pressure of all the gases combined; in other words the total pressure of the system.
the partial pressure is the fraction of which a certain gas is responsible (mole fraction * total pressure) so that all the 3 partial pressures combined make up the total pressure. (pCO + pCl2 + pCOCl2 = Ptotal)
if I calculate the Kp this way I get 3.37 as value (probably due to my approximation of Pa (for easy and fast calculating) as 1 atm = 10^5 Pa