Obtain about 45-500 mL of a 0.100 M acid solution, 45-50 mL of a 0.100 M solution of the salt of this acid, two Mohr (calibrated) pipettes, and four 50-mL beakers.  (The acid will also be 0.100 M in a strong electrolyte such as NaClO₄ in order to maintain a constant ionic strength in all the solutions measured.)  Using the calibrated (Mohr) pipettes, prepare the mixtures indicated in data table 17-A.  Pipette the indicated quantity of acid and salt into a labeled 50 mL beaker.  Since the pipettes are not calibrated to the tip, be careful to stop the flow of liquid in the calibrated region of the pipette.  Read the pipette accurately to the nearest hundredth mL, and record the volume on the data sheet. 

Note: You do not have to dispense precisely the quantities specified, but you do need to know what quantities you have dispensed in preparing the mixtures or your graph will be in error and your K_a will be incorrect!

Mix each solution thoroughly, being careful not to contaminate any of the solutions by transferring drops, on a stirring rod, from one solution to the next solution.  This can be accomplished by rinsing the stirring rod with distilled water and drying it before mixing another solution.  Use watch glasses to cover the solutions.

If not already done, open Experiment 27 from the “Chemistry with Computers” submenu of “Logger Pro.”  Check the layout of the computer screen for this experiment.  There is just a data table and a “live display” of pH. 

Your first job is to calibrate the probe.  Choose “Calibrate” from the “EXPERIMENT” pull-down menu.  The window “LabPro:1CH1:pH” will appear. Click on the window.  The “Sensor Settings” dialog box will pop up.  Under the “Calibrate” tab click on “Calibrate Now.” Remove the probe from the keeper solution (Please store the probe in the solution at all times except when it is being used.) and rinse thoroughly with distilled water from a squeeze bottle. Collect rinse solution into a 250-mL waste beaker.  Very gently blot the probe dry with a “Kimwipe”.  Immerse the probe in 20 mL of pH 4 buffer in a 50-mL beaker.  Enter the number 4.00 in the “Reading 1” data entry box.  Gently, use the probe to stir the solution continuously.  When the voltage value displayed under the “Choose Sensor(s) to Calibrate” stabilizes, click “Keep”.  Now rinse the probe again with distilled water, gently blot it dry, and immerse it in 20 mL of pH 10 buffer contained in another 50-mL beaker.  Enter the number 10.00 in the “Reading 2”data entry box.  Gently, use the probe to stir the solution continuously.  When the voltage value displayed under the “Choose Sensor(s) to Calibrate” stabilizes, click “Keep”.  Finally click on DONE to exit the “Sensor Settings” dialogue box.  The probe is now calibrated, however, if you quit or close the program at any time, the calibration will be lost and it will have to be repeated. This process goes faster if one person manipulates the probe and solutions while the partner operates the computer and records the values. If you need to start over, Click on the “Collect” button at the top of the screen. Select “Erase and Continue” from the “Erase Data? dialog box . Alternatively, “Clear All Data” command or “FILE”, “New” and OK the erasure of data

When you are ready to start, click the “Collect” button.  Place the pH probe in the first solution and stir gently until the pH is stable.  Click on “keep,” enter the sample identification letter, and then press the “Enter” key.  Repeat this process for each of your solutions.  It is best to keep your solutions in order.  Remember the glass electrodes are fragile and expensive.  Be sure to rinse the electrode with distilled water and dry with a Kimwipe, before reading each solution.  If the probe does not read correctly with the standard buffer or "behaves strangely," ask your instructor to check it.  After all of the solutions have been read, click "STOP" and then transfer the data from the computer’s data table to your data sheet.

Make the necessary calculations to complete both data sheets.

After completing calculations and recording pH’s for each of the four solutions on your data sheet, use the Graphical Analysis software to plot pH versus log [X-] / [HX]:

1. Open “Graphical Analysis” software
2. In data column “X” enter the calculated values for log [X-] / [HX]
3. In data column “Y” enter the corresponding pH values
4. Remember to change the headings for the X and Y columns.
5. Click Analyze, automatic curve fit, linear, OK, OK,
6. Print your graph (include your name, experiment number, lab section, date, unknown letter)
7. Record pH at log [X-] / [HX] = 0 (this will be the y intercept of your best line)
8. Use this pH to calculate Ka for your unknown acid

Submit your graph with your lab report.

Note: Be sure to answer the Post-lab Questions on Page 15-2 and submit them as part of your report. Also record the pK_a and K_a of your unknown acid on the instructor's data sheet.