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.
Propanoic Acid, , ionizes in water according to the equation above.  a) Write the equilibrium constant for this reaction b) Calculate the pH of a 0.265 M solution of propanoic acid. 
Part (a) is pretty straightforward. If you know how an equilibrium constant is written, you know how to do this! For (b),
  • We need to set up an ICE Table (Quick review I = Initial, C= Change, E= Equilibrium)
  • 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.