how do you find the Ka value for carbonic acid ?

Question

need it for a lab report, if you don't know the answer please set me on the right way

Answer 1

The reversible equilibrium equation is

H2CO3 === H2O + CO2

Ka = (H2)(CO2) / (H2CO3)

Since only the CO2 is a gas and the other components are liquid, you could simplify to

Ka = (CO2)

Answer 2

I'd have to look up the entire way to find it in my book, but I know for sure that you need to use the pH to find the pKa, and then the Ka is equal to either the -log of the pKa or 10 to the -pKa