# Equilibrium Constant Equation

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#### ratatat

##### Full Member
10+ Year Member
I just finished an EK test and I'm wondering one thing. The passage involved a concentration cell that is used as a pH meter.

They have two half-reactions

H2 + 2H2O --> 2H3O+ + 2e-
2H3O+ + 2e- --> H2 + 2H2O

The equilibrium equation is:

[H3O+]^2
_________

[H3O+]^2

I'm wondering why H2 isn't involved. Usually, aren't gases included in the equilibrium?

Thank you

in order to include the pressure of H2 in the equilibrium, you need the pressure of H3O+

since you are only dealing with concentrations here, the concentrations of H2, H2O and e- can be neglected.

in order to include the pressure of H2 in the equilibrium, you need the pressure of H3O+

since you are only dealing with concentrations here, the concentrations of H2, H2O and e- can be neglected.

Thank you for responding.

So you're saying because it's dealing with pH, the other concentrations don't matter? So if this question wasnt' dealing with pH, then would H2 be included?

Thank you for responding.

So you're saying because it's dealing with pH, the other concentrations don't matter? So if this question wasnt' dealing with pH, then would H2 be included?

if you weren't dealing with pH, then you would have the electric cell potentials of those half reactions, and use the nernst equation and other related equations to calculate the equilibrium constant. http://www.chem.usu.edu/~psilva/3600/14b.pdf