[Erica]

Ok, I've been struggling with this question for a few hours now, and its given me a lot of trouble:

For the rxn CaCO3 (s)-> CaO (s) + CO2 (g) : Delta H rxn= +177.5 kj/mol, and delta S rxn = 162.7 J/K mol. Assuming that these quantities do not depend on temperature, calculate the equilibrium partial pressure of CO2 at 825.5 K when the rxn takes place in a closed container originally devoid of any excess CO2 gas. (4 sig figs)

Ok, I've been using the equations G=Delta H rxn - T*delta S rxn, then taking that value and plugging it into
Delta G rxn=-2.3 RT log K

I know the K expression is [CO2]

Now...the correct answer is 1.850 * 10 ^-3

Why and how? (Thanks in advance for any help)

[Yggdrasil]

Using your procedure, I get the correct answer.  Are you remembering to check your units?  Remember that when you are calculating Delta G that Delta H is in kJ/mol but Delta S is J/mol K.

[Erica]

I think what I'm having a huge problem with is WHAT to plug into the equations. For the first equation, what T do I use? and for the second one, do I use the given T? That's the only thing that is stumping me right now.

[Yggdrasil]

Since you want to examine the reaction when it occurs at 825.5 K, you should plug in 825.5 K whenever you need to plug in a T for the equation.