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 protons. What shape should students expect for the titration of a polyprotic acid with strong base?
H3PO4(aq) + HO!(aq) ! H2PO!
4 (aq)+H2O(l), etc.
• Burettes need to be rinsed once with deionized water and once with the solution to be delivered. Air bubbles in the burette are your enemy!
• Be sure to get an accurate reading for the initial pH of your cola sample. Cola will be degassed and cooled in the stockroom—the bottles you’ll get will contain degassed cola. • Use precise (volumetric) glassware when transferring liquids during this experiment. Knowing exact volumes is critical.
• To save your titration data, bring a USB drive to lab.
Student Notes, Experiment 20! September 11, 2013
! 1Post-lab Report Rubric
I. Cover Sheet (5 pts)
• Lab partner’s name
• Honor pledge
II. Data & Results (55 pts)
A. Standardization of NaOH
Table 1. Volumes delivered and concentrations of “0.01 M” NaOH. (3 pts) Include mass of KHP, volume of “0.01 M” NaOH delivered, and calculated [NaOH] for all trials.
B. Titration of Cola Sample
Table 2. Initial parameters of titration. (3 pts)
Include volume of cola and initial pH of cola.
Table 3. Data for titration of cola sample. (12 pts)
Include volume of titrant, pH, change in pH (calculated), change in volume (calculated), ΔpH/ΔV (calculated), and average volume between points (calculated). Plot 1. Full page plot of titration curve and ﬁrst derivative of the titration curve. (20 pts) Include both the titration curve and its ﬁrst derivative on the same plot. Minor tick marks must be present on Volume and pH axes, and a secondary y-axis for the ﬁrst derivative must be present. Axes labels must be present for the primary and secondary axes. Lines showing how pKa was determined graphically must be present; use Microsoft Paint or your word processor’s drawing tool to add these.
Table 3. Ka data. (5 pts)
For both Ka’s, include the volume used for graphical Ka determination, pKa determined graphically, corresponding Ka, and true (“literature”) values for pKa and Ka. Include Ka1 calculated from the initial pH of cola and the initial concentration of acid. Sample Calculations
Standardized molarity of “0.01 M” NaOH solution, trial n: (3 pts) Initial concentration of phosphoric acid in cola sample: (3 pts) Ka1 calculated non-graphically, from initial pH and [H3PO4]: (3 pts) Percent dissociation of H3PO4 in cola: (3 pts)
III. Discussion (25 pts)
• 19 points for general discussion; 6 points for answers to discussion questions. • Include the objectives of each part of the experiment in one or two sentences at the beginning of the paragraph(s) devoted to each part.
Student Notes, Experiment 20! September 11, 2013
! 2• Report results actually obtained during the experiment, and discuss the theoretical basis of your results.
• Where your results differ from theoretical predictions, discuss speciﬁc sources of experimental error. Distinguish between systematic and random errors. • Address all the discussion questions (see below).
IV. 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)
Cite this Phosphoric Acid in Cola Drinks
Phosphoric Acid in Cola Drinks. (2016, Jul 12). Retrieved from https://graduateway.com/phosphoric-acid-in-cola-drinks/