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Determining OH- and pH of a weak base

  1. Jul 23, 2008 #1


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    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.
  2. jcsd
  3. Jul 23, 2008 #2


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    Staff: Mentor

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