Thought lab: Using turmeric as an indicator

This is a continuation of the Thought labs used as review for AP Chemistry. This style of review allows students to incorporate the skills they have acquired throughout their course experience in AP Chemistry while providing a platform for them to increase their comfort level in situations where they are asked to apply their learning to a new lab setting. While this review is designed to be done in a small group discussion, it can be easily adapted to allow students the oppor-tunity to work in the laboratory. Lime-A-Way (Problem 1), Catalysts and hydrogen peroxide (Problem 2) and Surface area and rate of reaction (Problem 3) were in the October, November and December 2017/January 2018 issues, respectively.

One of the challenges with these Thought labs is the limited materials list. This limitation helps students start to understand what they really need to measure. In some Thought labs, students will not need to measure anything but just compare two reactions.

Spend 7 - 10 minutes with your group working on the assigned lab problem. During that time you should formulate an “answer” to the proposed questions and be prepared to explain and justify your answer to other groups.

Turmeric, a spice and color agent in many types of cuisine, is added to yellow mustard for flavor and color. It changes color from yellow (acidic form) to red (basic form) at a pH of 7.4. Mustard also contains acetic acid. Given a sample of 0.50 M NaOH and some packets of mustard from Whataburger (or another local fast food restaurant), you are asked to create and perform an experiment to determine the mass percentage of acetic acid in mustard. You have the materials listed below available for your use.

Materials and chemicals list

• 2 packets yellow mustard               
• Small weighing dishes                      
• Electronic balance (±0.01 g)              
• 2 - 1 mL syringes (±0.01 mL)             
• 20 mL of 0.50 M NaOH                  
• 12-well reaction plate
• Lab apron
• Goggles
• Distilled water
• Toothpicks

Questions

1. Write a net ionic equation for the neutralization of the acetic acid by the NaOH.
2. List the measurements that you will need to make.
3. Show the setup for the calculations you will need to make.
4. On the graph paper, draw a sketch of the titration curve that you would get if you tracked the pH of the system as the reaction occurs.
   1. Mark the approximate equivalence point and explain why you chose that particular pH.
   2. Mark the location on the graph where you would have a buffer with the greatest capacity. Explain how you know this would be a buffer region.
   3. Explain your choice of the initial pH for the mustard.