pH, Equilibrium and AP Chemistry
The AP Exam looms closer and closer. As I start in earnest working on lots of problems with my face-to-face students, I plan to do the same here. Today, we are looking at a problem from AP Chemistry 2005. pH problems in AP Chemistry, along with a bunch of other topics especially equilibrium and Acids/Bases are among the most feared. Both with and without good reason. Remember, once you have the structure for a problem, the complexity automatically disappears. Here’s a snippet of that problem.
- Now, the equilibrium constant, becomes
- Plugging in values,
- Remember that in weak acids, x is very very small. How do we know that it is a weak acid? By the fact that it has an equilibrium constant. Strong acids are “single arrow” reactions, that means there is no equilibrium or equilibrium constant associated.
- So what does this mean? It means that 0.265 – x is approximtely equal to 0.265!! That is incredibly cool, because we just converted a messy quadratic into a LINEAR function.
- Let us now solve, or
- Time to put on units. M. CONTEXT: we are looking for H+.
- And… we all know that pH = -log [H+]. Plugging in, doing the math,
I know that many of you haven’t gotten to buffers yet, so I decided to truncate this problem with sections (a) and (b). This is the time to post questions, even if they are just about significant digits! Good luck, and ask away.