Chemical reactions ....
Carbon dioxide dissolves in water and reacts to a slight extent to make hydrogen ions and bicarbonate ions. It's something you hear (read) and remember.
CO2(aq) + H2O(l) <==> H+ + HCO3^- .................... Ka is small
A small value of the acid dissociation constant Ka indicates that the equilibrium lies far to the left, meaning that most of the CO2 in water STAYS CO2 and does not react.
Chemistry is not "all math and stuff", and you don't use math to figure out a chemical reaction. Judging from the caliber of your question, I'm afraid that much of what I wrote above may be lost on you.
You might be tempted to write (or someone may tell you to write)...
CO2 + H2O --> H2CO3
But if you did, it would be wrong, since there are no "carbonic acid" molecules in aqueous solution. They simply don't exist. What does exist in water are small amounts of hydrogen ions, H+, and bicarbonate ions, HCO3^-, and mostly just dissolved carbon dioxide.