Using the Nernst equation with a gaseous reactant/product

I have a question about using the Nernst equation.


$E = E^\varnothing + (0.059/n)(\log\frac{[Ox]}{[Red]})$


Specifically, I am interested in the reaction of $\ce{MnO4-}$ and $\ce{C2O4^2-}$ below:


$$\ce{2MnO4- + 5C2O4^2- + 16H+ = 2Mn^2+ + 10CO2 + 8H2O}$$


How can I write Nernst equation for $\ce{CO2/C2O4^2-}$ in order to determine the titration curve of this reaction?. I'm confused because $\ce{CO2}$ isn't in the solution, and therefore there isn't a concentration for it.


Is it right if I remove $\ce{CO2}$ from the equation? For example:


$$E = E^\varnothing + \frac{0.059}{2}\left(\log\frac{1}{[\ce{C2O4^{2-}}]}\right)$$




**HINT**


If we consider a reaction such that :


$$\mathrm{n^-\alpha^+Ox^+ + n^+\beta^-Red^- = n^+\alpha^-Ox^- + n^-\beta^+Red^+}$$


Where $\mathrm{Ox^+/Red^+}$ is the redox couple at the positive side of the battery and $\mathrm{Ox^-/Red^-}$ correspond to the negative side redox couple, then can show easely,


$$e=E^+-E^-=e^\circ+\frac{RT}{n^+n^-F}\ln\left(\frac{a\_{Ox^+}^{n^-\alpha^+}\cdot a\_{Red^-}^{n^+\beta^-}}{a\_{Ox^-}^{n^+\alpha^-}\cdot a\_{Red^+}^{n^-\beta^+}}\right)$$


Where $e^\circ$ is the stardart electromotrice force.


Now you just need to know what is the activity of a constituant when it is a gas or something else. In this [answer](https://chemistry.stackexchange.com/a/65372/15235) you'll find some good informations. 


Sorry in advance if there are typo but I think no.


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