[sickness]

I got this problem in which I need to find PH we are given Pco2 of 10^-2.5 and temp of 15 degrees.  My problem is that I am having trouble in understanding Henery's law in order to find the Kco2.  If anyone can help me with the Kco2 and how I can derive it I know that the rest of the problem will be cake.  Thanks for any help on this.

[Demotivator]

You don't post enough data to derive Kco2.
So, I assume the constant Kco2 as well as Ka (for H2CO3) are to be gotten from a table.

Basically, CO2 dissolves in water (to form H2CO3) by henry's law: [H2CO3] = Kco2 x Pco2
[H2CO3] can be calculated and PH can be determined according to:
H2CO3 <-> H+  + HCO3-   Ka