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.


&

 

[Blueshift5]

Based on the information provided, it seems like you have correctly set up the equilibrium expression and solved for the concentration of OH-. However, the concentration of OH- should be 1.9 x 10^-5 M, not just 1.9 x 10^-5. The units for concentration are usually given in moles per liter, so the final answer should be [OH-] = 1.9 x 10^-5 M. This means that the pH of the solution would be 4.72, calculated by taking the negative log of the [H+] concentration, which can be found by using the equation [H+] = Kw / [OH-], where Kw is the ionization constant for water (1.0 x 10^-14 at 25°C). So, in summary, the [OH-] of the solution is 1.9 x 10^-5 M and the pH is 4.72. If you are still having trouble, it may be helpful to double check your calculations and make sure you are using the correct units.