Citric Acid Concentration of Strawberry Fanta (or Barq's Rootbeer) as Determined by pH Measurement

In a previous experiment, you determined the concentration of citric acid (hereafter known simply as CA) by titration. Titration has a few flaws, one of which is that if we are trying to titrate a solution that has a color, 'seeing' and endpoint is difficult. Just 'seeing' is the other big problem, because the 'end point' is a judgment call.

In this experiment, we are going to solve both problems at one, by measuring the pH of the soda sample and using our knowledge of equilibrium calculations, we can determine [CA] without every looking at it, just measuring it with some objective instrument (a pH meter)

Now, nobody said the math was going to be easy, in fact, we are going to cheat. Citric acid is a tri-protic acid, and doing a pH calculation of a X.Y molar solution is really hard because we would have to solve three equilibrium equations simultaneously, which nobody wants to do, so we are going to calculate a pseudo equilibrium constant, (Ka*) the one for this equilibrium:

H₃C₆H₅O₇  ↔ H⁺(aq) + H₂C₆H₅O₇^-(aq)

Which, I know looks like the Ka1 expression for citric acid (it is) but we are going to pretend that this is the ONLY equilibrium going on. (a safe assumption because we make the assumption throughout the experiment. Any error because of it is actually canceled out.

You are going to make solutions of known initial citric acid concentration (think 'I' in ICE) by adding pure (s) citric acid diluted into a 25.00 mL flask. The pH will then measured allowing us to calculate the equilibrium (think 'E' line in ICE) concentration of H+(aq) allowing us to calculate the value for Ka*. Three good runs, of course.

Armed with that value, we can measure the pH of the soda sample, and the problem turns into one of the 'other' kind of equilibrium problems (Given pH and Ka, calculate initial acid concentration)

Lab Report: Yes, you should do one. Not sure what flavor... Ask me in lecture.

Pre-Lab questions: (worth 8 points)
1. HZ is a weak acid. An aqueous solution of HZ is prepared by dissolving 0.035 mol of HZ in sufficient water to yield 1.0 L of solution. The pH of the solution was 3.93 at 25.0 °C. The Ka of HZ is __________.
2. The pH of a different solution of HZ is measured to be 4.23. What is the concentration of this new solution (you are trying to determine that pesky 'I' line here)