[truediarist]

Hi all,

I'm trying to wrap my head around a relatively simple equilibrium question relating gas equilibrium constants Kc and Kp, specifically with dissolved (aqueous) solutions reacting with gases. How do they relate when you have a simple hemoglobin interaction such as

Mb(aq) + O2(g) <=> MbO2(aq) when only O2 is the gas?

I realize kc and kp are related by the ideal gas law and as such kp = kc * RT ^(delta n), but my main confusion is when my textbook says that

Kc = [MbO2] / [Mb][O2] and because O2 is a gas, you can also say
Kc = [MbO2] / [MB]*(O2) where O2 is in partial pressures.

Isn't the two different by a factor of RT by the ideal gas law? (see textbook as seen from Google: http://i.imgur.com/o8BUd.png)
Thanks!

[ttran]

HI truediarist
This reaction contain both gas and aqueous spices so I don't think you can use that relationship. Instead, keep in mind that in the equation Kc=[MbO2]/[Mb]*[O2], 3 term [MbO2], [Mb] and [O2] are all unit-less, which mean they don't have unit (the reason why Kc is constant  ). The reason for that is because each term in the equation of Kc is equal to its concentration divide by its concentration in standard state (let say the standard state is 1 molar).
 Therefore in general, [O2] term also express the ratio and doesn't has any unit, and it should be equal to the term partial pressure because for the partial term, you also have to divide it by the pressure in standard state and it is 1 atm.

Hope it's help