PH chemistry Help! General Chem

Caculate the pH of ta buffer solution that contains 0.405 M HCOOH and 0.326 HCOONa. The ka for HCOOH is 11.8*10^-4...
Please tell me step by step what to do.

Thanks!

I think you wanted to write Ka = 1,8 * 10^-4.

Concentrations:
[HCOOH] = 0,405M = Ca
[HCOONa] = [HCOO-] = 0,326M = Cs

From the dissociation of formic acid:

HCOOH + H2O <-------> HCOO(-) + H3O(+)

you'll have

[H3O+] = Ka * ( Ca / Cs ) = 2,236 * 10^-4M -----> pH = 3,65

Bye Bye :)