Determining OH- and pH of a weak base

[dnt]

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.

 

[Borek]

This is so called amphirotic substance. You have to take into account both dissociation and hydrolyzis. See:

http://www.chembuddy.com/?left=pH-calculation&right=pH-amphiprotic-salt

for pH calculation details.


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