What is the ph of an aqueous solution with a hydrogen

A.) What is the ph of an aqueous solution with a hydrogen ion concentration of [H^+]= 5.0 x 10^-9 M?

B.) A monoprotic acid, HA, disassociates:

The equilibrium concentrations of the reactants are products are

[HA]= 0.270 M
[H^+]= 3.00 x 10^-4 M
[A-] = 3.00 x 10^-4 M

Calculate the Ka value for the acid HA.

Ka=