Posted by Douglas on Tuesday, May 10, 2011 at 5:58pm.
In the titration of a weak base with a strong base, the equivalence point occurs on the acid side of 7.0. That tells you that 5,4,3, and 2 must be wrong and that the correct answer is #1. However, if you wish to calculate it that can be done, too. The pH at the equivalence point is determined by the salt, in this case NH4Br. The NH4^+ is hydrolyzed to
NH4^+ + H2O ==> H3O^+ + NH3
Ka = (Kw/Kb) = (H3O^+)(NH3)/(NH4^+)
You know Kw = 1E-14.
Kb = 1.8E-5 (for NH3)
(H3O^+) = x = (NH3)
(NH4^+) is about 0.05 if you titrate 0.1M HBr and 0.1M NH3. Work it out. I get (H^+) = 7.45E-6 and the H = about 5.13. That falls just about half-way between the limits of 4.4 and 6.2.
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