Looking for some ideas on approaching acid-base equilibrium?
I’ve had this lesson plan stashed away for a rainy day and – looking outside – it seems like the right time to share this AP and Honors Chemistry outline. Please read and feel free to comment on what you find useful from this and from your own experiences.
Jeff Goodwin, Content Director, Science
The key to solving these problems quickly is to start using the concept of Ve (equivalent volume). First one should always determine the Ve for any of the types (Strong-Strong, St-Weak, Wk-St, and don’t bother with wk-wk (they’ll never have it on the exam)). Start off by using that good old formula: MaVa=MbVb. Depending what information has been given in the problem, either Va or Vb will be the Ve. I’ll give examples of each. Students must already have knowledge of net-ionic equations, ICE tables, knowledge of the strong acids and strong bases, Hend.-Hass. Equation, Ka and Kb, conjugates, and stoichiometry. By the way I’m not going to use sub and superscripts in order to save time.
Each type (see below) can be broken down into the following:
1. before any titration has started
2. after titration has started, but before Ve
3. at Ve
4. after Ve
OK, let’s start with the different types:
The easiest type to do (of course). I’ll follow the numbering system I’ve used above in all of these. In all cases calculate Ve first.
1. Before titration begins. Simply calculate the pH as you normally would for the strong acid or base.
2. Set up an ICE table with the net ionic (H+ + OH- – H20) Enter the values into the ICE table (keep with molarities here) and calculate=
3. At Ve, pH=7 (of course!)
4. Use the same drill for after Ve as you did in #2 above.
Strong Acid-Weak Base
Now the fun begins! Solve Ve first! Here we go.
1. Before titration begins (say you only have a weak base present). Simply solve pH by using the Kb of the weak base.
2. Set up ICE table with net ionic, enter the values in MOLES (saves a bunch of time). This will be a buffer situation—use Hend.-Hass. Equation to solve for pH. A SPECIAL NOTE: 1/2Ve will mean pH = pKa of the weak conjugate ACID.
3. At Ve: pH does NOT = 7. At this point, all of the weak base has been consumed and you only have the weak conjugate acid present. Solve pH using Ka of the weak acid. Remember, you’ve been using moles in the ICE table—you have to convert to M using the TOTAL volume.
4. Only consider the EXCESS strong acid (the amount beyond Ve) in MOLES. Divide by the TOTAL volume to get M of H+ and then solve for pH.
Weak Acid-Strong Base
Basically the same drill as was just presented. All you have to remember is to deal with Ka instead of Kb and vice-versa. My suggestion: Only have the kids learn one form of Hend.-Hass. And have them “burn” it into their brain cells. Also, the equation can be used with moles instead of M. This really saves a lot of time. The only thing you have to constantly remind them of is to convert back to M when solving for the pH or pOH.
Weak Acid-Weak Base
Do not worry about this titration—will never be on the exam. You would have to compare the relative strengths of the weaks using Ka and Kb. You’ll never see this at this level.