UTC: Monday, February 18th, 2019

Math, buffers and H-H, revisited in the new curriculum

I think that there are (unfortunately) some useful, simple math things to know when it comes to buffers and H-H in the new AP chemistry curriculum.

Obviously, it has always been crucial to know that when the ratio of weak acid to conjugate base (or weak base to conjugate acid) in a buffer solution is 1:1, then pH = pKa, i.e., that log(1) = 0, but in the context of the new curriculum I think that other useful data points include;

log(10) = 1, i.e., that pKa + 1 = pH when the ratio of salt to acid = 10:1

log(0.1) = -1, i.e., that pka – 1 = pH when ratio of salt to acid = 1:10.

There may be others (in your opinion), but the CED specifically references these in EK 6.C.2 b. (also see d).

I prefer to calculate things fully, but since LO 6.18 exists, AND the EK statements noted above also exist, I think those pointers are handy to know.

The use of the FULL H-H equation is also still on the table IMO, i.e., one having to compute the specific pH given a set of concentrations for the two buffer components, or perhaps at a specific point in a titration. I say that for two reasons;

a. because the H-H equation is still on the equations & constants sheet, and

b. because I can’t seem to let it go just yet.

However, I will concede it is difficult to tie it to a specific LO, which is what HAS to happen, if it is to appear as an AP exam question (maybe LO. 6.16???? – I’d like to hear other opinions on this, perhaps we can forget about this too). What has been taken off the table (via one of the exclusion statements), is a question like 1993, 1(c).

Speak Your Mind

*