AP Chemistry Laboratory #18: Buffering in Household Products. Lab days: Wed. and Thurs., March 21-22, 2018 Lab due: Friday, March 23, 2018

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1 AP Chemistry Laboratory #18: Buffering in Household Products Lab days: Wed. and Thurs., March 21-22, 2018 Lab due: Friday, March 23, 2018 Goals (list in your lab book): The goals of this lab are to experiment with buffers and determine if household products exhibit buffering activity. Background (DON T write in your lab book): Buffers are solutions that resist changes in ph when acids or bases are added. In order to accomplish this, a buffer must contain both an acidic and a basic component. These two components should not neutralize each other but be available to neutralize hydrogen ions or hydroxide ions from other sources. One way to carry this out is to combine a weak acid-base conjugate pair, such as acetic acid and acetate ions or ammonium ions and ammonia. An acetic acid-acetate ion buffer can be prepared in several ways. One can combine a solution of acetic acid and sodium acetate; one can start with a solution of acetic acid and neutralize some of it with sodium hydroxide; or one can begin with a solution of sodium acetate and partially neutralize it with hydrochloric acid. By varying the type of weak acid or base and changing the concentration ratio of the conjugate acid-base pairs, buffers can be made for any ph range. It is possible to derive an expression to calculate the ph from the equation for the dissociation of a weak acid: HA (aq) H + (aq) + A - (aq) weak conjugate acid base The acid-dissociation constant expression is then: Solving the expression for [H + ] gives the relation: This expression shows that the hydrogen ion concentration (and thus the ph) depends on two things: the value of the acid-dissociation constant (K a ) and the ratio of the concentrations of the acid and conjugate base. It is possible to calculate quantities needed to prepare a solution of a known ph by choosing an acid whose dissociation constant is somewhere near the desired [H + ] and by solving the equation to find the correct ratio of [HA]/[A - ]. Notice that if the concentrations of conjugate acid and base are the same, the hydrogen ion concentration equals the value of the K a. An alternative expression can be derived by taking the negative logarithm of both sides of the equation:

2 This equation can be simplified. Since -log[h + ] = ph and -log K a = pk a, we obtain This equation can also be written In this equation, [base] is the concentration of the conjugate base, and [acid] is the concentration of the conjugate acid. Because the acid and base are in the same solution, their volumes are identical, and the equation can also be written in terms of moles of base divided by moles of acid. This equation is known as the Henderson-Hasselbalch equation and is frequently used to calculate the ph of buffers, especially by biologists and biochemists. Since the relationship between K a and K b for a conjugate acid-base pair is K a x K b = K w = 1.0 x then, by taking the negative logarithm of both sides, pk a + pk b = 14 The equation for a basic buffer can be written Research questions (answer in complete sentences; please don t write or number the questions): 1) The ph of a solution is measured to be Calculate the hydrogen ion concentration of the solution. 2) Give the conjugate acid or base of the following: (a) conjugate base of HC 2 H 3 O 2 (b) conjugate acid of CN - (c) conjugate base of HSO 4 - (d) conjugate acid of CO 3 2-3) What is a buffer? How is a buffer prepared? 4) A buffer solution was prepared using the conjugate acid-base pair acetic acid and acetate ions. Write the chemical equations showing the reactions that take place when (a) H + ions are added to the buffer solution (b) OH - ions are added to the buffer solution 5) Write the neutralization reaction (in net ionic form) for the titration of acetic acid (CH 3 COOH) with sodium hydroxide (NaOH). [Research continues on the next page!]

3 6) Look at the figure below: (a) Which titration curve represents the titration of a strong acid with a strong base? Which curve represents the titration of a weak acid with a strong base? Do either of these curves display a buffer effect? Explain. (b) What is meant by the term equivalence point? What are the approximate equivalence points for curves (A) and (B) in Figure 2? (c) At what point along a titration curve is pk a equal to the ph for a weak acid? Materials - Day 1 (don t list in your lab book): 5 ml 0.1 M HCl 1 25 ml graduated cylinder 5 ml 0.1 M sodium hydroxide (NaOH) 4 10 ml graduated cylinders 20 ml 0.1 M ammonium chloride (NH 4 Cl) 3 droppers 20 ml 0.1 M sodium acetate (NaC 2 H 3 O 2 ) 1 permanent marker 20 ml 0.1 M ammonia (NH 3 ) 1 stirring rod 20 ml 0.1 M acetic acid (HC 2 H 3 O 2 ) ml beakers 4 10 ml beakers 1 test tube brush 4 50 ml beakers 1 ringstand 1 Lab Quest with ph probe 1 test tube clamp 8 ml ph 4 buffer solution 8 ml ph 10 buffer solution Hazards (list in your lab book): (include the safety contract and the hazards of acetic acid, ammonia, sodium hydroxide, ammonium chloride, hydrochloric acid, and sodium acetate) Procedure - Day 1 (do NOT list in your lab book): 1) Physically and chemically clean the glassware. 2) Calibrate the ph probe. 3) Test the ph of water before and after adding HCl a) Obtain 5 ml of 0.1 M HCl in labeled 10 ml beaker b) Obtain 5 ml of 0.1 M NaOH in labeled 10 ml beaker c) Add about 20 ml of distilled water to a 100 ml beaker d) Test the ph of the water e) Add one drop of 0.1 M HCl to the water g) Test the ph of the water again h) Repeat by adding a second drop of HCl and a third drop of HCl

4 4) Test the ph of water before and after adding NaOH a) Add about 20 ml of distilled water to a 100 ml beaker b) Test the ph of the water c) Add one drop of 0.1 M NaOH to the water d) Stir the solution e) Test the ph of the water again f) Repeat by adding a second drop of NaOH and a third drop of NaOH 5) Measure the ph changes of an acetic acid-acetate ion buffer solution a) Obtain about 20 ml of 0.1 M HC 2 H 3 O 2 in a 30 ml beaker b) Obtain about 20 ml of 0.1 M NaC 2 H 3 O 2 in a 30 ml beaker c) Prepare a buffer by using 10 ml graduated cylinders to add 10 ml of 0.1 M HC 2 H 3 O 2 and 10 ml of 0.1 M NaC 2 H 3 O 2 to a 100 ml beaker d) Measure the ph of the buffer solution e) Add 1 drop of 0.1 M HCl g) Measure the ph of the solution h) Repeat 5e-g for a second drop of 0.1 M HCl i) Repeat 5e-g for a third drop of 0.1 M HCl j) Repeat 5c to prepare a fresh sample of acetic acid-acetate ion buffer k) Repeat 5d-i with 0.1 M NaOH instead of the HCl 6) Measure the ph changes of an ammonia-ammonium ion buffer solution a) Obtain about 20 ml of 0.1 M NH 3 in a 30 ml beaker b) Obtain about 20 ml of 0.1 M NH 4 Cl in a 30 ml beaker c) Prepare a buffer by using a 10 ml graduated cylinders to add 10 ml of 0.1 M NH 3 and 10 ml of 0.1 M NH 4 Cl to a 100 ml beaker d) Measure the ph of the buffer solution e) Add 1 drop of 0.1 M HCl g) Measure the ph of the solution h) Repeat 6e-g for a second drop of 0.1 M HCl I) Repeat 6e-g for a third drop of 0.1 M HCl j) Repeat 6c to prepare a fresh sample of ammonia-ammonium ion buffer k) Repeat 6d-i with 0.1 M NaOH instead of the HCl 8) Clean up! Post-Lab Questions - Day 1: 1. How did the addition of a strong acid affect the ph of distilled water? 2. How did the addition of a strong base affect the ph of distilled water? 3. (a) How did the acetic acid-acetate ion buffer affect the ph changes with HCl? (b) Write a chemical equation to show the reaction of HCl with the buffer. (c) How did the acetic acid-acetate ion buffer affect the ph changes with NaOH? (d) Write a chemical equation to show the reaction of NaOH with the buffer. 4. (a) How did the ammonia-ammonium ion buffer affect the ph changes with HCl? (b) Write a chemical equation to show the reaction of HCl with the buffer. (c) How did the ammonia-ammonium ion buffer affect the ph changes with NaOH? (d) Write a chemical equation to show the reaction of NaOH with the buffer. On day 2, you will be exploring the behavior of a household product to determine if it exhibits buffering activity. You will titrate a solution of the household product to determine whether it acts as a buffer. You will need to plot the ph vs. the amount of titrant added. The two titrants provided for your use will be 0.1 M NaOH and 0.1 M HCl. NOTE: Your household product may have to be diluted in order to have the provided titrants work. Your product should be at a

5 concentration where it can be titrated with 50 ml of titrant (or less!). Suggestions of starting concentrations to try are undiluted, 1 ml undiluted:10 ml distilled water, and 1 ml diluted:100 ml distilled water, but you may use any dilution that is successful (as long as you record it exactly in your data). A few quick titrations (say, recording every 5 ml) will let you determine a good concentration to use for your graphable (carefully recorded) trial. Materials - Day 2: (List the specific materials you plan to use in two columns.) Procedure - Day 2 (List at least 5 numbered steps of the procedure you plan to follow.) Post-Lab Calculations/Questions - Day 2: 1. Graph the ph vs. volume of titrant for each of your Day 2 trials on graph paper [or on a computer that you can print from] (y-axis = ph, x-axis = volume of titrant added in milliliters) and tape the graph(s) into your lab book. 2. Identify the equivalence point and half-equivalence point on your graph. 3. Did your household product exhibit buffering activity? Why do you think so? (Be sure to label your graph with the region where buffering was shown if you think there was buffering.) 4. If your household product exhibited buffering activity, use the following table (and the ingredients from the label and/or other resources) to determine which buffer was likely present: Be sure to explain your reasoning regarding the buffer/ingredient you chose. Lab handout based on the experiment Preparation and Properties of Buffer Solutions in Laboratory Experiments for Advanced Placement Chemistry (Second Edition) by S.A. Vonderbrink (Flinn Scientific, 2006) and Investigation 15: To What Extent Do Common Household Products Have Buffering Activity in AP Chemistry Guided-Inquiry Experiments: Applying the Science Practices (College Board, 2013)