[anubistsai]

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.

[DevaDevil]

p_CO V = n_CO R T; this will give you n_CO (mind the units; use m³ and Pa if you use the common gas constant of 8.3145 J / mole K)
p_CO = partial pressure of CO

furthermore, you know 3e-2 moles of PURE COCl₂ were put in; so:
n_{start COCl2} = 3e-2

n_CO = n_Cl (for every CO formed, there is also a Cl₂ formed) = calculated above
n_{end COCl2} = n_{start COCl2} - n_CO (every mole CO formed means 1 mole COCl₂ less

this will give you the molar numbers, and thus the total amount of moles.
n_total = n_CO + n_Cl + n_{end 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:
K_p = partial pressure of COCl₂ / partial pressure of CO * partial pressure of Cl₂

[anubistsai]

Answer still doesn't make sense.    What is this total pressure that you were referring to?

[DevaDevil]

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. (p_CO + p_Cl2 + p_COCl2 = P_total)


if I calculate the K_p 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