[cmaunder]

Hello all,

I have what I hope is a fairly fundamental question.  I have a closed system (a 500ml bottle) containing air and (for simplification) pure water (250ml air, 250ml water).  My interest is in determining how much dissolved inorganic carbon (DIC = H2CO3 + HCO3 + CO3) will be formed in the water due to equilibration with CO2(g) in the air.  The air was added by a very short exposure to the atmosphere and is therefore identical in all respects to atmospheric air, while the water initially contains no DIC. 

I have a very clear understanding of how this works in an open system, because the partial pressure of the CO2(g) is constant at atmospheric levels, which makes things easy.  I know that in a closed system, as equilibration proceeds, the CO2(g) is drawn down into the water and the partial pressure of CO2(g) decreases.  What I don't know is given that the partial pressure is not constant and thus the Henry's Law equation becomes unwieldy for determining the initial step of CO2(g) <-> CO2(aq) equilibrium, how do you go about determining exactly how much total DIC will be formed in the water. 

I've been going through all the chem texts I own from undergrad for a couple days now and still nothing, so any help is greatly appreciated.

[Borek]

the partial pressure is not constant and thus the Henry's Law equation becomes unwieldy

Huh? It works perfectly, as usual.

You will need to add equation for mass balance of CO₂ - whatever was initially in the air is now split between air and water. That will give you two equations, in two unknowns.