Experiment 20: pH Titration: Phosphoric Acid in Cola Drinks
Post-lab Assignment or Report
The post-lab report for this experiment is due at the beginning of the following lab period. Student notes for the lab will be available on the lab T-Square site. Learning Objectives
Students will be able to...
• Use a known mass of solid acid to determine an unknown concentration of a basic solution (this process is called “standardization”).
• Execute a titration using good, reliable technique.
• Use stoichiometry calculations and reactions occurring during a titration to convert from a known mass of acid to an unknown amount of base (and concentration). • Observe a pH indicator to determine the endpoint of a titration of acid with base (or vice versa).
• Identify important points and regions on a titration curve, including buffer regions and equivalence points.
• Calculate the concentration of an acid (with known identity) in a sample using data from a titration with strong base.
• Calculate Ka of an acid (with known identity) in a sample using titration data. Talking Points for Pre-lab Discussion
Demonstrate the typical setup for a titration (including mounted pH meter). Turn the burette’s stopcock with care!
Discuss the process of standardization, which will be used to determine the exact concentration of a “0.01 M” NaOH solution.
Introduce potassium hydrogen phthalate (KHP), a weak acid used to react with added NaOH.
Draw and discuss a titration curve for this standardization (titration of weak acid with strong base). You won’t actually measure this titration curve, but it is very instructive! Draw the ﬁrst derivative of the curve on the same plot.
What is the goal of standardization? What important situation exists at the endpoint? How will the endpoint of the titration be detected? Discuss the use of phenolphthalein as a pH indicator.
Introduce the idea of a polyprotic acid, and illustrate how phosphoric acid (H3PO4) can lose multiple...
References: (5 pts)
1. Look up true values for the Ka’s of phosphoric acid. Discuss the validity of using graphical
analysis to determine the Ka’s for phosphoric acid. (3 pts)
2. Why is it necessary to degas the cola sample prior to the titration? (3 pts)
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