the question is simply to determine the [OH-] and pH of a solution that is .210 M in HCO3- first of all if i can get [OH-] then i can easily get pH by converting [OH-] to [H+] and then doing the -log[H+] so that part is easy. i just cannot get [OH-] what i did was first write the equation: HCO3- + H20 --> H2CO3 + OH- and use the Kb value for HCO3- which I looked up to be 1.7 x 10^-9 then used .21-x for the concentration of HCO3- and x to be the concentrations of both H2CO3 and OH- then using the Kb equation I got: x^2 / (.21-x) = 1.7 x 10^-9 solving for x = 1.9 x 10^-5 shouldnt that be [OH-]? but its not correct. can someone show me what mistake i made? thanks.