This is an in-class activity that I use in my advanced general chemistry course to teach students how to qualitatively assign oxo anions as non-basic, feebly basic, or basic. Being able to qualitatively make these assignments helps students when we get to predicting solubility of compounds using Bronsted acidity and basicity.

Students are generally comfortable calculating pK_b from the charge of the oxo anion and the number of oxo groups and then comparing the result to a table in their textbook to assign basicity, but struggle to make this assignment qualitatively without looking at the textbook. In the course as a whole, I emphasize the power and importance of being able to qualitatively predict trends in reactivity, in part to provide an alternative approach to the fall semester of the course--which focuses on quantitative aspects of physical chemistry--and in part to prepare students for organic chemistry, where they can't simply plug a product into an equation and generate a synthesis.