We're back!

Yesterday in class we reviewed for tomorrow's exam.

Another question:
What is the Kb of a base if 500.0mL of a solution containing 0.153 mol of the base and 0.191
mol of its conjugate acid has a pH of 6.114? Over what pH range would this conjugate acid/
conjugate base pair make an effective buffer?
How do you determine the pH range that the solution would be a good buffer?
A weak conjugate acid/conjugate base combination will be an effective buffer over a pH range that is within 1 unit of the pK_a of the weak conjugate acid, so we have to work through to find the K_a. Plugging the numbers from the problem into the Henderson-Hasselbalch equation,
6.114 = pK_a +log(0.153/0.191)
pK_a = 6.210
NOTE: You can calculate concentrations of the conjugate acid and base, but the "500.0mL" volume mathematically cancels out, so you can just use mols directly in the H-H equation for this specific problem.
At this point, we can answer the buffer range question. Since the pK_a of the acid is 6.210, this system will make good buffers from 5.210 to 7.210.
Once you know the pK_a of the conjugate acid, the pK_b of the conjugate base is:
14 - 6.210 = 7.790
And the K_b of the base is:
10^-7.790 = 1.623x10^-8

Mastering Chemistry Assignments 12 & 13:
A number of people have talked to me about the 2 Mastering Chemistry assignments that were due last week. I realize that the sandbagging efforts and evacuations caused some difficulties. At the same time, these assignments were posted to Mastering Chemistry and announced on the class blog before classes were cancelled so everyone should have had the opportunity to work on them before the due date. As a compromise, I have changed the grading settings in Mastering Chemistry so that the possible points on those assignments is decreased by 5% for every day they are late. This means that you can still recover most (but not all) of the points on these assignments even if you do them late.