What is ka for h3bo3(aq) h plus (aq) plus h2bo3-(aq)?

The equilibrium constant expression for the reaction you provided would be ( K_a = \frac{[H^+][H_2BO_3^-]}{[H_3BO_3]} ). However, the specific value of ( K_a ) for this reaction would depend on the concentrations of the species involved in the particular experimental conditions.