The pH of Solutions
In this experiment, you will use a pH probe connected to the Serial Box Interface. Your instructor will demonstrate how to calibrate the pH probe using Logger Pro. One feature of Logger Pro that you will likely want to use this week is the Digital Meter. When measuring the pH, keep the following in mind: While measuring the pH of acidic solutions, the pH probe should be calibrated in an acidic range (4-7). While measuring the pH of basic solutions, the pH probe should be calibrated in a basic range (7-10).
You will also be using serial dilutions in this experiment. What is a serial dilution? Consider this example: In Procedure 1 below, the 0.010 M HCl would be prepared from the 0.10 M HCl and the 0.0010 M HCl would be prepared from the 0.010 M HCl. When using serial dilutions, you should always be conscious of keeping your glassware clean. Otherwise, a common mistake is that a solution prepared later in the dilution sequence may accidentally become too concentrated.
You are expected to work in pairs to make your measurements, but you may combine a couple of groups to speed the preparation of your solutions. If there is an odd number of people in the lab, then one group of three will be allowed.
Procedure 1
Prepare 50.0 mL of each of the following solutions by serial dilution from the stock HCl
solution. Then measure the pH of each solution.
| a) 0.10 M HCl | b) 0.010 M HCl | c) 0.0010 M HCl |
Prepare 50.0 mL of each of the following solutions by serial dilution from the stock NaOH solution. Then measure the pH of each solution.
| a) 0.10 M NaOH | b) 0.010 M NaOH | c) 0.0010 M NaOH |
Analysis 1
Prepare a table that shows the expected pH and the actual pH of each of the solutions
above. Account for any variation.
Procedure 2
Prepare 50.0 mL of each of the following solutions by serial dilution from the stock HC2H3O2
solution. Then measure the pH of each solution.
| a) 0.10 M HC2H3O2 | b) 0.010 M HC2H3O2 | c) 0.0010 M HC2H3O2 |
Prepare 50.0 mL of each of the following solutions by serial dilution from the stock NH3 solution. Then measure the pH of each solution.
| a) 0.10 M NH3 | b) 0.010 M NH3 | c) 0.0010 M NH3 |
Analysis 2
Did the dilutions of acetic acid and ammonia yield a predictable change in pH? Is the
change what you expected? Explain.
Procedure 3
Prepare 50.0 mL of each of the following solutions. Then measure the pH of each solution.
| a) 0.10 M NaHCO3 | b) 0.10 M NaC2H3O2 | c) 0.10 M NH4Cl |
Analysis 3
Rank all of the 0.10 M solutions used in today’s experiment from lowest pH to
highest pH. Compare the actual rank to the rank that was expected (Tro, Table 15.12,
Chemistry: A Molecular Approach,
1st ed., p.
692). Account for any variation.
Final Analysis
Prepare a table that shows the expected pH and the actual pH of each solution used in
today’s experiment. You will need to use calculation methods found in Chapters
15-16 of
Tro. Account for any variation in your results.
Laboratory Report
You may choose to submit an individual or
group report for this lab. Follow the guidelines for
Laboratory Reports located at
http://webs.anokaramsey.edu/chemistry/Chem1062.
For this lab report, you will need to include a
title, procedure, results, discussion, and references.
The discussion should
provide reasons for the observations made during the lab and the results of the
various tables. Potential errors and their effects should also be included.
Follow your instructor’s directions for submitting this lab report. Remember to name the file as specified (Lastname_pH or Lastname1_LastName2_pH). If you submit a paper copy of your report, include one set of sample calculations for one of the trials as an appendix for the expected pH of each of the seven different substances -- HCl, NaOH, HC2H3O2, NH3, NaHCO3, NaC2H3O2, and NH4Cl, either handwritten or typed. Microsoft Equation 3.0 objects placed into a Microsoft Word document produce neat, organized equations. If you are emailing your report, use the subject line “Chem 1062: pH Lab”. You may embed Excel tables into the Word document so that the professor may view the formulas in each cell to see how you calculated the expected pH's. If you use Word tables, you will need to include the sample calculations in an appendix as well. If you worked in pairs and are submitting this assignment on an individual basis, please underline your own name and include your lab partner’s name on the assignment.
Lab written by Lance S. Lund, Anoka-Ramsey Community College. Updated April, 2011.