Chemistry 14BL :LQWHU

Vinegar A Study of a Weak Acid and its Buffer The procedure for this assignment is found on pages 44-46 of your lab manual. A handout will be provided to replace the final paragraph on page 45 and the buffer preparation component of week 2 of the assignment. The study questions, and the guidelines for the notebook preparation, and notebook report for both weeks of the assignment are given in this document. Due Dates: Laboratory Preparation Week 1 (Individual) Feb 17 - 20 Laboratory Preparation Week 2 (Individual) Feb 24 - 27 Laboratory Report Week 1 (Individual) March 3 - 6 Laboratory Report Week 2 (Group) March 3 - 6

Laboratory Preparation (Week 1) Study Questions On separate lined paper, complete the following questions. Staple these to the copy of your prelab notebook work and turn them in at the start of the lab period. 1. The printed label on a bottle of commercial vinegar states that the acetic acid

concentration is 5%. (a) If the manufacturer had reported two significant figures in the concentration, what

range of values would round to 5.0 %? (b) Calculate the concentration in molarity of the upper and lower values from (a).

The molecular weight of acetic acid is 60.05 g/mol. How many digits can you report in your answers?

(c) What assumption did you make to calculate the concentrations in part (b)? (d) What is the percentage difference in the concentrations you calculated in part (b)?

Explain what value you chose for the denominator in this calculation?

2. Acetic acid is a monoprotic weak acid with a pKa of 4.74. (Ka = 1.8 x 10-5) (a) What is the pH of 10 mL of a 5.0% solution? (b) What is the pH of the solution if you now add 30 ml of water to solution (a)? 3. How will the equivalence point volume differ if you titrate the two solutions in

question 2? Explain your answer.

Technique Preparation (I) On-line Technique Video and Resources Go to the following Web site and click on the appropriate title to download the video. http://www.oid.ucla.edu/edtech/impcastvideos Select course related and then the titles below. Use of a Buret http://ccle.ucla.edu Calibration of the Sartorius pH Meter

IMPORTANT: You should review the concepts of acid-base chemistry from 14A for this experiment. A problem set addressing acid-base chemistry is posted on the course website. (II) Read section II.6, pages 10-11 in your lab manual on the theory behind the pH meter and print the instructions on the use of the pH meter posted in the Assignments tab of the course website. (Take this set of instructions to lab with you.). Notebook Preparation* Prepare your lab notebook for the experimental work involved in determining the concentration of a commercial vinegar product. Specifically, prior to your lab section meeting, your notebook should include the following sections of the report: Introduction: Include the goals of the both parts of the assignment as well as the experimental techniques you will use to attain those goals. Procedure References: Appropriately reference all primary and supplemental printed materials and any digital sites that you will use for the assignment. Flow Chart: Prepare a flow chart of the assignment procedure for Week 1 including sufficient detail that you can use it as a checklist during the assignment to monitor your progress at each step along the way. In preparing your flowchart place an asterisk beside each chemical for which you provide safety information in the next section of the report. MSDS Information: In tabular format provide the following information for NaOH (0.5M) and CH3COOH (5% by weight/volume, which you will work with in the assignment: ** Every page of your notebook must include the title of the assignment, your name and section, and the date that the work was done, or the report written. Always select the MSDS site that gives you information closest to the concentration or grade of the chemical you will be working with in the assignment.

product name chemical formula formula weight melting point; boiling point and density health hazard data (summarize in your own words spill and disposal procedures (summarize in your own words

Note: Start a NEW page in your notebook for the remainder of the prelab assignment. The previous material will be turned in at the beginning of the period; these pages will be turned in at the end of the lab period. Data Tables: Set up (1) a data table to show the calculation of the dilution that you make from the storeroom standard stock solution (leave a space to record the actual concentration provided) and (2) a table to record your first set of titration data. (In lab you will prepare other titration data tables as needed.) For each titration table set up a column for pH and another for volume of titrant. Allow space at the top of the table to record the volume of the pipetted aliquot of acetic acid and the precise concentration (with error) of the standard NaOH you prepared in (1).

In-class Work Week 1 Complete the dilution table and the titration data tables.

Post-Lab Report for Week 1 Your notebook report will include: Data - An orderly record of your data (printouts of data tables from EXCEL are

acceptable) - Your data table should include all the raw data, which was turned in on the day the

work was done, as well as all the calculated values for the derivative graphs - Each data table should have a title - Label all data entries clearly Graphs (If you did 3 trials, you will have a total of 9 graphs) For each trial, you need a full titration graph, a titration graph of just the equivalence point region and a first-derivative graph of just the equivalence point region. The latter two graphs should have scales that allow you to interpolate the equivalence point to the precision of the data (that is you need to be able to read the x-axis on these graphs to 0.02 mL) Your graphs should have - correct scales which will require you to use all of the page - labeled axes with units - appropriate titles - an indication on the expanded graphs of the equivalence point volume for EACH

trial. - an indication on the full titration graph of the pKa (vinegar). This corresponds to the

pH when half the volume of titrant required to reach the equivalence point has been added.

Data Analysis

Concentration - From the equivalence point volumes from the titration graphs, the volume of acetic

acid titrated, and the concentration of your diluted NaOH standard solution, calculate the concentration of the unknown vinegar solution for each trial.(Remember to subtract the initial buret reading from the equivalence point volume determined from your graph if your initial buret reading is not 0.00 mL)

- Calculate the average concentration from your trials pKa - Using the values for the pKas marked on the full titration curves, determine the average pKa from your set of titrations. Make sure you label the pKa clearly on each

of the full titration graphs. Error Analysis - Calculate the relative average deviation for the concentration of your vinegar solution - Calculate the % inherent error for ONE of your vinegar titrations - Calculate the % difference between your molar calculated concentration of the

vinegar solution with the manufacturers stated concentration of 0.83 M. - Calculate the average deviation for your pKa. Report this as an absolute error Conclusion - Summarize your results of the concentration and its relative error and the pKa and its

absolute error. - Compare your percent relative average deviation with the percent inherent error. - How does the difference between your average concentration and the manufacturers

stated concentration compare with your calculated percent relative deviation and calculated inherent error?

Laboratory Preparation (Week 2)

Study Questions On separate lined paper, complete the following questions. Staple these to the copy of your prelab notebook work and turn them in at the start of the lab period. 1. Calculate the pH of a buffer solution prepared by mixing 20 mL of 5.0% acetic acid

with 20 mL of 0.50 M NaOH. (pKa for acetic acid = 4.74; Ka for acetic acid = 1.8 x 10-5) 2. Calculate the pH of a buffer solution prepared by mixing 20 mL of 5.0% acetic acid

with 20 mL of 0.50 M NaOH and 100 mL of water. 3. Calculate the equivalence point volume for the titration of 10 mL of 5.0% acetic acid

with 0.50 M NaOH 4. Calculate the equivalence point volume for the titration of 10 mL of 5.0% acetic acid

and with 0.50 M NaOH and 100 mL of water. 5. Calculate the volume of 0.20 M NaOH required to bring the pH of 20 mL of 5%

acetic acid to 4.74, the pKa of acetic acid. Notebook Preparation for this continuing assignment should include: Flow Chart: A brief flow chart summary of the key procedures of Week 2 of the assignment MSDS Information: In tabular format provide the following information for HCl (0.5M), which you will work with in the assignment:

product name chemical formula formula weight melting point; boiling point and density health hazard data (summarize in your own words spill and disposal procedures (summarize in your own words

Note: Select the MSDS site that gives you information closest to the concentration or

grade listed above for HCl. Data Tables: On a new page set up (1) a data table to indicate who your partners are for this part of the assignment, who is responsible for preparing the buffer solution, who is responsible for each of the four sets of buffer titrations. Then set up (2) a table for titration data similar to the ones you prepared for Week 1. Include a place in the table heading to indicate the specific buffer titration conditions that you will perform as well as the concentration of the titrant.

Note: Be sure to obtain an Excel copy of the data from each of your partner(s) It is important that each person has a complete set of data at the time the report is submitted. At the end of the lab period in Week 2, each person will submit a copy

Always select the MSDS site that gives you information closest to the concentration or grade of the chemical you will be working with in the assignment.

of the data he or she acquired. Thus, the TA will have an original copy of all of your groups work.

Post-Lab Report for Week 2 THIS IS A GROUP REPORT. ONLY ONE REPORT IS SUBMITTED FOR THE GROUP. The post-lab report will include: Experimenters/Authors - Write the names of your group members and their responsibility in the experiment

and in writing this post-lab report Data (printouts of data tables from EXCEL are acceptable) - Organize data into tables and label each data table clearly with title. Your data table

should include all the raw data as well as all the calculated values for the derivative graphs.

Graphs (Use EXCEL to plot the titration graphs) There should be one full graph and one full derivative graph for each set of aliquots - Prepare a full titration graph AND a first derivative graph for the combined buffer

titrations for the two sets of 5-ml aliquots of vinegar with HCl and NaOH. - Prepare a full titration graph AND a first derivative graph for the combined buffer

titrations for the two sets of 10-ml aliquots of vinegar with HCl and NaOH. Your graphs should have: - correct scales and full use of graph paper - labeled axes with units - appropriate titles and labels Analysis - Determine the points on the combined buffer graphs that correspond to the limits of

the buffer region. Using the value for the pKa determined from week 1 and the pH of the solution at those points of the titration, calculate the ratio of [OAc-]/[HOAc] at these points. (This is the range of pH values in which the Henderson-Hasselbach equation is valid.)

Discussion and Conclusion - Compare the combined titration curves for the data obtained in Week 2 with the full

titration curve obtained from data acquired in Week 1. How are they similar or different?

- Summarize all the results