Teacher S Name: XXX

Developed by: Andrew Young and BSS staff

Grade Level: 10-12

Title: Acids & Bases in the Home

Adapted from:

• “How much Acid is in Juices and Soft Drinks”, AP Chemistry Lab Manual, Investigation Four.
• “Acid-Base Properties of Household Products”, Investigating Chemistry Through Inquiry. By Volz, D. and Smola, R.

Applicable NGSS and other Standards:

HS-PS1-2. Construct and revise an explanation for the outcome of a simple chemical reaction based on the outermost electron states of atoms, trends in the periodic table, and knowledge of the patterns of chemical properties.

11-12.WHST.7Conduct short as well as more sustained research projects to answer a question (including a self-generated question) or solve a problem; narrow or broaden the inquiry when appropriate; synthesize multiple sources on the subject, demonstrating understanding of the subject under investigation.

objectives

In this Investigation you will,

• Design an experiment to test variables affecting the pH of various juices or sodas.
• Use a Vernier pH probe to collect data for all trials.
• Perform a Titration to determine the concentration of H+ ions in a common juice or soda.
• Use Stoichiometric calculations to analyze deviations from expected results.

MATERIALS

The materials will be available for students to use as they see fit to design their individual experiments. Students need to identify which materials they will specifically be using during the lab.

Labquest setup

Helpful Hints

1. If students want to use dark soda or juice, redirect them or ask them what problems they may have seeing the color change at the end of the titration. This will save time and frustration. Doesn't pH sensor get around colorometric procedures? Is it too advanced to suggest a dilution to overcome this?
2. Do not add NaOH directly near the probe's sensor. Use a clamp to position probe and buret on opposite sides of beaker.
3. To reach equilibrium quickly, the use of a stir plate is suggested, otherwise stirring with a glass rod can be substituted. Ensure stir bar does not contact pH probe.
4. When entering data points during titrations:

a.Press Collect to start the Run.

b.In the graph viewing screen, press Keep each time you want to enter a data point, i.e. after every 5 or 10 drops or after specified volume increments.

c.When entering # of drops or volume increment into the Labquest, be sure to enter as a total volume added. For example, rather than saying 5, 5, 5… Enter as 5, 10, 15, 20…

1. When students see their first larger change in pH, they should begin adding drop by drop, or using lower volume increments. This will make for a smoother curve with a more distinct inflection point.
2. Try to have students use lower volume samples if possible. This will allow for more trials and quicker titrations.

PROCEDURE -

Experimental Design -

1. Ask the students to think of a testable question, provided it relates to acid concentration in juices or soft drinks. They are to propose a hypothesis to answer their question, design a procedure to collect and analyze data, and make conclusions to their question.
2. Explain that students will be provided with a list of materials from which they can choose to conduct their experiment.
3. For materials that they choose, they are to consult the MSDS sheets for those chemicals to identify safe handling procedures and any precautions that must be taken during the experiment. MSDS sheets can be found at:
4. In their Procedure section, they are to sketch a plan for their equipment including labels of all equipment.
5. All data and observations must be recorded for all trials throughout the experiment.

TITRATION –

1. Accurately measure 25 - 50ml of juice or soda and pour into one of the Erlenmeyer flasks or beakers.
2. Choose an indicator that you believe will help visualize when the equivalence point (moles acid = moles base) is reached. Use the table below to make your decision about which indicator to use.

1. Using the pH probe, measure the starting pH of the liquid and record in the data table.
2. Fill the 50ml Buret with 0.10M NaOH, and record the starting volume in the buret.
3. Turning the stopcock, add ~0.5 mL of NaOH to the flask and record the pH again. Be sure to be as accurate as possible when measuring the amount of NaOH added each time. This volume will be entered into the Labquest each time you press Keep.You should be able to estimate to the nearest 0.01 mL. Note and record the color of the indicator at each point as well.
4. Continue adding NaOH to your flask, measuring and recording pH until your pH begins to change rapidly between additions. At this point, slow your addition to one quick turn of the stop cock until the pH reaches ~12.
5. After the pH has reached ~12, empty out your flask, rinse it thoroughly, and begin the process again. Titrate each of your liquids at least twice.

DATA and analysis (Example)

GRAPH –

Graph your data on LoggerPro or the LabQuest and identify the inflection point (the point at which the graph changes direction). Use the volume of NaOH added at the inflection point to do the Analysis questions.

Analysis Questions -

1. Write the balanced chemical equation for the reaction between NaOH and the primary acid identified by the ingredients on the juice/soda label, i.e. – Citric acid in Lemon-Lime Soda.
2. Using Stoichiometry, calculate the molarity (concentration) of the acid in the juice/soda before the titration. Be sure to include units and all steps in the calculation.
3. Calculate the pH of the juice/soda using the concentration found in Question 2.
4. Looking at your graph, generally speaking, what happens to the pH of the juice/soda as the volume of titrant increases?
5. Share your results with three other lab groups. What are some similarities and/or differences that you see with other lab groups.

POST LAB ASSESSMENT –

1. How might the results be different if someone used the graduated cylinders to measure the volume of soda versus using a volumetric pipette? Would this affect the significant figures in your answer?
2. Your lab partner rinsed the buret with water between trials, but not with the titrant. How might this affect the calculated pH of your juice/soda? Why?
3. If you titrated a cola, like Coke or Pepsi, what were some problems you encountered when recording the color of the indicator? If you didn’t titrate a cola, ask another group who did how they solved the problem of the indicator?
4. Review your graph of pH versus volume of base added. Where did the indicator change color for each trial? What was the pH associated with the color change? What assumptions can you make regarding the pH at this point and the shape of your titration curve? Use a diagram to show what you think might be happening at the molecular level.
5. Imagine you were a dentist and a patient of yours was showing significant tooth decay. When asked how much sugar they consume, they said they only drank diet soda and orange juice. What might you suggest to them about how to help their teeth.