Idea Transcript
AMHS AP Chemistry
Buffer Lab
Ch. 15
Prelab Questions: 1) What does it mean to say that for most weak acids, the percent dissociation is less than 5%? 2) Using the chart below, rank the following equimolar solutions from lowest to highest pH: KHSO4 , NaHCO3,Ba(OH)2, KNO3, HClO4 Na2CO3, HC2H3O2, HCN, Li2SO3, NaHSO3. -6 3) If the Ka for an acid is 5.2 x 10 , what is the pKa? 4) If the pKa is 4.50, what is the Ka? 5) What is the best method of selecting a buffer solution for a desired pH range? Table 1 Acid Formula Ka1 Ka2 pK a1 -1 Iodic HIO3 0.77 1.7 x 10 Sulfurous
H2SO3
Bisulfate Acetic
HSO4HC2H3O2
-2
1.7 x 10 1.2 x 10-2
-5
1.8 x 10
-7
Carbonic
H2CO3
4.3 x 10
Hypochlorous
HCIO
3.0 x 10
Hydrocyanic
HCN
4.9 x 10
-8
-10
-8
6.4 x 10 -
-11
5.6 x 10 -
pK a2 -
1.77
7.19
1.92 4.74
-
6.37
10.25
7.52
-
9.31
-
Chemicals: Unknown solid weak acid, glacial acetic acid (HC2H3O2), NaOH solution (about 0.1M), phenolphthalein solution (1%), sodium acetate (NaC2H3O2). Equipment: burette, beaker, Erlenmeyer flask, graduated cylinder, pH paper or pH meter. Safety Alert: Acids and bases are harmful to skin and eyes. Wash spills off skin with lots of water. Phenolphthalein is dissolved in alcohol, so it is flammable. Keep the solution away from flames. Wear Chemical Splash Goggles. Procedure – Part I: Determining the Ka of a Weak Acid 1. Measure out a small quantity of the solid acid to be tested, (about 0.2 g). It is not necessary to know the exact amount. 2. Fill your burette ½ full with the prepared base solution. (No need to standardize the burette this time!) 3. Measure precisely 50.0 mL of distilled water into a beaker, add the acid, stir to dissolve and mix well. 4. Pour exactly 25.0 mL of the acid solution into an Erlenmeyer flask. Add 1-3 drops of phenolphthalein to the acid solution in the Erlenmeyer flask, and then titrate using the NaOH solution from your buret. (No need to record the volume this time!) Stop adding the NaOH when the first pink color persists throughout the solution for at least 25 seconds. 5. Pour the contents of the solution you just titrated in the flask back into the beaker from step #3 above and mix well. 6. Take a small sample of this mixture to your instructor for pH testing. Record the value for your data analysis section. pH = _____________ Procedure - Part II: Buffers 1. You will now make a buffer solution using the glacial acetic acid and the solid sodium acetate. Your goal is to create 250 mL of a solution with a pH = 4.95 and a concentration of less than 0.250M. (Make certain you record the calculations and procedure used to generate this solution for your lab write-up.) Test the pH of your solution on the pH meter and record. pH = _____________
AMHS AP Chemistry Buffer Lab Ch. 15 2. Make a “serial dilution” of the glacial acetic acid to make 100 mL of a 0.100M solution. You must begin this serial dilution with 5.00 mL of concentrated acid and use no more than 25 mL of each subsequent acid sample in each dilution to obtain the final concentration. Each dilution cannot exceed 250 mL of total solution. 3. Replace the sodium hydroxide standard in your buret with the class standard (record the actual concentration listed on the container). 4. Measure out exactly 25.0 mL of the dilution from #2 and place it in an Erlenmeyer flask. Begin to titrate the acetic acid solution from #2 by adding approximately 7-10 mL of standard in “one pour” (your choice of volume…but accurately record the amount you add). Remove a small sample of this solution for pH testing (do not discard…return this sample to the flask for further titration). pH = ______________ 5. After the pH testing in #4: your goal is to now create a buffer solution with a pH = 5.10 by adding more base to the 25 mL sample in #4. (Make certain you record the calculations and procedure used to generate this solution for your lab write-up [the Ka for acetic acid is 1.8E-5].) Test the pH of your solution on the pH meter and record. pH = ______________ Disposal: The solutions may be washed down the drain with a large excess of water. Data Analysis Part I 1. Determine the Ka value for the unknown solid acid. Make certain you detail how you are able to complete this task! (Calculate your percent error by obtaining the identity of the unknown acid and finding the accepted Ka value in the literature.) 2. Now, using the Ka value from above, calculate the pH of the untitrated solution just prior to combining it with the titrated solution in step #5 of part I. Part II 1. Detail your calculations for creating the buffer solution for #1. 2. Show your work for the serial dilution of the glacial acetic acid. 3. Determine the expected pH for the solution after your addition of 7-10 mL of base to the solution and compare to the actual pH recorded in #4. 4. Detail your calculations to create the pH 5.10 buffer solution in #5 and compare the expected to actual pH of your resulting solution. Post Lab Questions: 1. Assume the acid dissociation constant for the acid salt NaHSO4 is to be determined. (a) Write the chemical equation to show this salt ionizing in water. (b) Write the chemical equation showing the anion acting as an acid in water. (c) Write the equilibrium for the acid dissociation of the anion. (d) Explain the procedure for determining the Ka using the expression from (c). 2. Why is it not necessary to know the exact mass of the acid whose Ka is to be determined? 3. Why is it not necessary to know the exact concentration of the NaOH solution used or standardize the buret? 4. Why is it necessary to precisely measure the volume of water used to dissolve the acid? 5. Write the Henderson-Hasselbalch equation and show how it can be solved to find the Ka of an acid when the concentrations of a conjugate acid and base are equal. 6. A buffer solution for a certain bacteria is needed for addition to some growth media. The bacteria has a narrow pH tolerance zone from 4.21-4.25 and produces acetic acid as a by-product of its metabolism. Your lab is stocked with the chemicals in Table 1 above. Are you able to create a buffer solution? Which chemical would you use and how would you prepare 1L of this buffer solution?
AMHS AP Chemistry
Buffer Lab
Ch. 15
Notes for your lab notebook write-up: • Begin the lab write-up with the prelab. • Background on Ka; buffer solutions; and the methods of creating a buffer solution at a particular pH. • Add a section on writing your procedure after the Hypothesis. • Procedure Part I & II: Detail your procedures (including the glassware you used) for these steps. Make certain you clearly demonstrate the concentrations and movement of material for your serial dilution • Data: Construct a data table with all of the pH information clearly labeled based on the solution. (i.e. pH based on the titrated and untitrated solution – Part I) • Data Analysis: Examine the section on the lab sheet. ¾ MAKE CERTAIN ALL OF YOUR CALCULATIONS ARE DETAILED!!! ¾ Unknown acid is potassium hydrogen phthalate: Ka = 3.9E-6 Convert your Ka value into a pKa and this known value into a pKa for your error analysis. ¾ Work your way back through the procedure…any step requiring a calculation must be shown. ¾ Your serial dilution steps must be detailed…show the concentrations between the starter solution (17.4M) and your final concentration (M1V1 = M2V2 would be appropriate). ¾ Post Lab Questions go in this section. • Sketch a graph of the pH (y-axis) vs. concentration for your final procedure. (The serial dilution…addition of base…addition of base….what would your endpoint pH be?) Label the areas similar to our notes…4 sections. • Normal Conclusion with two errors discussed!