This is a simple in class exercise to review acid- base equilibria and to lead the students to thinking about metals as both Lewis and Bronsted- Lowry acids. I use it as a discussion starter when I introduce the role of metals in biological acid/base chemistry in my upper division inorganic course, but it can be used at any level once acid-base equilibria has been covered.
A student should be able to relate a pKa to a Ka and calculate from that the concentration of components at equilibrium. A student should be able to relate the magnitude of Ka to the strength of the acid.
Primary assessment is based on the quality of the ensuing discussion. Secondary assessment can be done by asking a related but different question on a exam.
Once students (at the jr/sr level) "remember" how to do acid base equilibria, this leads into a very nice discussion of the different ways that metal ions can affect the acidity of a system.