acid equilibrium constants?

Question

did i get this right?

at 25 C, a 0.025 M solution of formic acid HCOOH, is found to have a hydronium ion [H3O+] concetration of 2.03 x 10^-3M. Calculate the ionization constant of formic acid.

.0812

thanks!!!

Answer 1

From definition of Ka,
[H+] [HCOO-]/[HCOOH]= Ka,
From reaction equation, H+ and Formate ion come from partially dissociated acid, so
[2.03x10-3][2.03x10-3]/[0.025-0.00203]=Ka
This look like about 2x10^-4 to me.