Question

What is the pH of rainwater at 30C in which atmospheric Co2 has dissolved, producing a constant [H2Co3] of 1.40x10^-5 M? Take into account the auto ionization of water.

Im very confused on how to even start this problem

Answer 1

This looks like a rather complete discussion.
http://www.pwtag.org/researchdocs/Used%20Ref%20docs/52%20Carbondioxide%20in%20water%20equilibrium.pdf