In your lab notebook, start with a fresh right-hand page, and write the title and purpose of lab 1. Include also your name, your lab partner’s name and the date. Don’t forget to update the table of contents at the front of your lab notebook (the pages should also be sequentially numbered).

Alwaysdouble-space writing in the notebook (that is, leave an empty row between lines of text, including rows in tables).

You may cut and paste the “Skills” through “Procedure” section of the lab (make sure it is a neat job of cutting and pasting, else you can write it in longhand). This is a good general format to follow for the order of sections. Following this, begin a “Data” section in which you will write many tables. All handwritten table cells should contain two rows, even if you are only expecting one number; one row is for the heading of each column (with units). Tables that are print-outs should be neatly taped into the notebook.

Follow that with the “Analysis” section and then the “Conclusions” section. After that, please write an abstract...this one is not hard, since it’s a “fill in the blanks” type (see format at the end of this handout).

Photocopy the lab 1 pages or tear out only the carbon-copy pages. Make sure the title and name are legible. This lab is dueWednesday, April 10 in class.

Chemistry 163

Lab 1: Determination of Ka of a weak acid using two methods

Purpose: To determine the value of the dissociation constant for a weak acid using two volume-measuring methods: the drop counter and the buret.

Skills:• Use of a buret and drop counter for precision volume measurement

• Use of a digital pH meter for quantitative determination of the acidity/alkalinity of a solution

Chemical equation:HA + OH– H2O +A– which is a neutralization but this isn’t really the equation that the dissociation constant equation is based off. On second thought, please write the chemical equation that your calculation of Ka will be based.

Chemicals needed:• 2 M unknown acid (record the letter of the acid)

• 1 M NaOH

• distilled water (dH2O)

Equipment needed:• Labquest data logger and digital pH meter

• Drop Counter, with ringstand and clamps

• Several large test tubes

• 150 mL and 250 mL beakers

• 10 mL graduated cylinder

• Magnetic stirrer (but not the stir bar in your drawer)

Special equations needed:[H+] = 10–pH

Procedure:

1. Set up and calibrate the drop-counting apparatus, as stated in the “Using a Drop Counter with pH probe” (attached at the end of this handout – steps 1 through 5).

2. Record the exact concentration of the acid and the sodium hydroxide (units, too) in the data section of your notebook.

3. Set up the experiment by performing steps 6 and 7 of “Using a Drop Counter with pH probe” but before starting step 8, call the instructor over for a check of the apparatus. Do not proceed without the instructor’s okay.

4. Run the experiment (steps 8 through 10). It is vital that you transfer the data to a flash drive; make sure (either by opening the file on the Labquest or opening it on a laptop) that the data have been successfully transferred. Record the name of the data file; use a better name than the default name the Labquest assigns it. You will be generating a fair number of Labquest files this quarter.

5. Obtain two burets, one for water and the other for the sodium hydroxide solution. Rinse each with the solution it will contain, and discard (in the waste container) the rinse. Set up the burets using the buret clamp on a ringstand; label the burets so you know which holds what substance. Make sure you know how to read a buret.

6. Fill one buret with water (that is, up to the 0.00 mL mark) and the other with the sodium hydroxide solution. Make sure that the stopcocks are closed and do not leak.

7. Using a 5 mL calibrated pipet, dispense 5.00 mL of the unknown acid solution into each of four clean labeled (#1 through #4) large test tubes.

8.Complete the preparation of each test tube by adding the following volumes from the appropriate burets.

Solution / Volume of unknown acid (mL) / Volume of NaOH (mL) / Volume of distilled water (mL)
#1 / 5.00 / 2.50 / 7.50
#2 / 5.00 / 5.00 / 5.00
#3 / 5.00 / 7.50 / 2.50
#4 / 5.00 / 10.00 / 0.00

9. Mix the contents of each test tube well with the glass stirring rod.

10. Record the pH of each of the four solutions. Make sure that you are rinsing the pH probe with distilled water in between measurements. Check with another group to make sure that you are getting sensible numbers.

Data

Of course, record the exact concentrations of the unknown acid and NaOH, down to the last significant figure.

Depending on the number of data points generated by the drop counter, print out and tape into your notebook this data. Since it is a table, make sure that you’ve edited the file enough to give each column a heading with units.

Append the table from step 8 of the procedure with a pH column and tape that into the notebook here.

Analysis

First, write the full mathematical expression for calculating Ka.

For the drop counter data, choose five points (not clustered together – get some from low pH, middling pH and high pH) and generate an analysis table showing the concentrations of the various species needed to calculate Ka. The last column should be Ka for each row.

Generate a plot of volume of NaOH dispensed (x-axis) versus pH (y-axis) and tape it into your notebook. Make sure the table contains units and a title (“Titration of an unknown acid with NaOH” is a fair title). Do not connect the data points with a smooth curve.

For solutions #1 to #4, generate an analysis table showing the concentrations of the various species needed to calculate Ka. The last column should be Ka for each row.

Show the calculationsnecessary for one of the rows in each table.

Calculate the average Ka using the drop counter, and calculate the average Ka using the burets.

Enter your initials, the letter of the unknown acid used, and these averages in separate columns on the spreadsheet on the computer at the front of the classroom. Calculate, for each column, the class average and standard deviation (for your lettered acid, of course).

Conclusion questions

1. What would happen, quantitatively, to your Ka if your forgot to add water to the test tubes, but recorded the final volume as 15.00 mL anyway?

2. The calculations are done assuming that the OH– added is reacting completely with the acid. Suppose this were not the case (that not all of the OH– added reacted completely); how would this affect Ka? Show a brief calculation that demonstrates how Ka would change.

3. Describe the shape of the graph of the titration (drop counter) experiment. Did the pH change abruptly or smoothly?

4. The equivalence point of an acid/base titration is the point where the number of moles of acid being titrated is equivalent to the number of moles of base added. Where was this point achieved in this experiment? Perform a calculation to determine the volume of base added at this point, then find the data point on the graph that corresponds to your calculation. What pH is found at the equivalence point? Is this a surprising result or not, and why?

5. In the abstract, I ask you to compare the two volume-measuring methods (drop counter vs. buret). How can you tell if these two methods are giving you equivalent results? How can you tell if one method is “significantly” better than the other? Don’t give waffly answers; quantify the criteria.

Abstract

Rules for all abstracts:

• Should be short (<125 words), word-processed on a separate sheet of paper (so not part of your lab notebook), double-spaced, and proofread (the Loft in the library is willing to proofread all student materials, tell them Tracy sent you). Staple the abstract sheet to the front of your lab notebook copy or photocopy.

• The format of the abstract should always have your name first, followed by your partner’s name (if they helped with the lab). The next line should be the title of the lab.

• The abstract should be a summary of the results: ideally, you can do this in a few sentences for experiments in this class. My style is that the first sentence should be a summary about what experiment you did and the (major) method or equipment setup that you did it with. The next sentences should be a statement of the major result, either the new product molecule you made or the major numerical result of the experiment. The final sentences should state how robust the result is (i.e., the confidence question) and what significant errors may have occurred. Note that this is my style; if you read Chemical Abstracts, you will see that scientists’ styles differ but that the information I suggest above is always given.

• If you have not learned to do so already, please figure out how to use superscripts, symbols and subscripts so you can write “Ka = 2.44  10–8 “.

For this experiment, word-process the following and fill in the details as you go along:

Your name and your partner’s name, North Seattle Community College

A COMPARISON OF TWO METHODS FOR THE DETERMINATION OF THE DISSOCIATION CONSTANT FOR (NAME OF ACID)

The dissociation constant of the weak acid (chemical formula of acid) was determined in two ways by quantitatively adding aqueous sodium hydroxide and measuring the pH of the resulting solution. Using a drop counter to measure volumes, the Ka of this acid averages (give the mean of your drop counter results). Using burets to measure volumes, the Ka of this acid averages (give the mean of your buret results). We conclude that (the methods yield equivalent results/one method (specify) is significantly better because…). We are (confident/not confident) of these results, since the class value was (give the mean and standard deviation of the class values), and our result (fell/did not fall) within two standard deviations of the class mean. (Optional sentence if you are more than two standard deviations away, giving a possible error source).

Using a Drop Counter with pH Probe

  1. Lower the Drop Counter plate onto a ring stand and clamp it into position (see Figure 1).
  2. Connect the Drop Counter to LabQuest DIG 1
  3. Obtain the plastic 60 mL reagent reservoir. Close both valves by turning the handles to a horizontal position.
  4. Follow the steps below to set up the reagent reservoir for the titration.
  5. Rinse the reagent reservoir with a few mL of the NaOH solution and pour the rinse NaOH into an empty 250 mL beaker.
  6. Use a utility clamp to attach the reservoir to the ring stand. (see Figure 1).
  7. Fill the reagent reservoir with ~ 60 mL of the NaOH solution. Note: Record the precise concentration of the NaOH solution in your data table.
  8. Place the 250 mL beaker, which contains the rinse NaOH, beneath the tip of the reservoir.
  9. Drain a small amount of the NaOH solution into the 250 mL beaker so that it fills the reservoir’s tip. To do this, turn both valve handles to the vertical position for a moment, then turn them both back to horizontal.
  10. Discard the drained NaOH solution in the 250 mL beaker as directed.
  11. Calibrate the drops that will be delivered from the reagent reservoir.
  12. On the top row of the LabQuest toolbar, open the SENSORS menu and choose Calibrate ! DIG 1: Drop Counter.
  13. Place a 10 mL graduated cylinder directly below the slot on the Drop Counter, lining it up with the tip of the reagent reservoir.
  14. Open the bottom valve on the reagent reservoir (vertical). Keep the top valve closed (horizontal).
  15. Click the Calibrate Now button.
  16. Slowly open the top valve of the reagent reservoir so that drops are released at a slow rate (~1 drop every two seconds). You should see the drops being counted on the screen.
  17. When the volume of the NaOH solution in the graduated cylinder is between 9 and 10 mL, close the bottom valve of the reagent reservoir. Note: Do NOT adjust the top valve for the remainder of the experiment.
  18. Enter the precise volume of NaOH (read to the nearest 0.1 mL) in the edit box.
  19. Click OK
  20. Discard the NaOH solution in the graduated cylinder as directed, and set the graduated cylinder aside.
  21. Assemble the apparatus. (See Figure 1.)
  22. Place the magnetic stirrer under the Drop Counter.
  23. Connect pH Sensor to CH 1 at the top of the LabQuest.
  24. Remove the storage bottle from the electrode by first unscrewing the lid, then removing the bottle and lid. Thoroughly rinse the lower section of the probe, especially the region of the bulb, using distilled or deionized water.
  25. Insert the pH Sensor through the large hole in the Drop Counter.
  26. Attach the Microstirrer to the bottom of the pH Sensor. Rotate the paddle wheel of the Microstirrer, and make sure that it does not touch the bulb of the pH Sensor.
  27. Adjust the positions of the Drop Counter and reagent reservoir so they are both lined up with the center of the magnetic stirrer.
  28. Ensure reagent reservoir has a sufficient amount of NaOH solution.
  29. Add 10 mL of unknown acid solution to a 150 mL beaker.
  30. Add 50 mL distilled water into beaker.
  31. Lift up the pH Sensor, and slide the 150 mL beaker containing the acid solution onto the magnetic stirrer. Lower the pH Sensor into the beaker.
  32. Adjust the position of the Drop Counter so that the Microstirrer on the pH Sensor is just touching the bottom of the beaker.
  33. Adjust the reagent reservoir so its tip is just above the Drop Counter slot.
  34. Turn on the magnetic stirrer so that the Microstirrer is stirring at a fast rate.
  35. You are now ready to begin collecting data. Press . No data will be collected until the first drop goes through the Drop Counter slot. Fully open the bottom valve. The top valve should still be adjusted so drops are released at a rate of about 1 drop every 2 seconds. When the first drop passes through the Drop Counter slot, check the data table to see that the first data pair was recorded.
  36. Stop the titration if the pH reaches 13.0 or if 50 mL NaOH is added. Turn the bottom valve of the reagent reservoir to a closed (horizontal) position, then press.
  37. Repeat steps (6) to (8) if more than one run is required.
  38. To save data:
  39. Insert a FAT32 formatted USB drive or SD card into the top of the LabQuest.
  40. On any screen use the stylus to tap on the word file at the top of the screen.
  41. Tap EXPORT to save your file as a .txt file.
  42. Tap the picture of the USB or SD card to enter that device.
  43. Tap on word Untitled at the top of the screen and use the backspace key on the onscreen keyboard to change the file name.
  44. This text file can later be exported into Excel for data analysis.
  45. To finish using the LabQuest:
  46. To turn off the machine hold down the power button until the screen goes black.
  47. If you get an unsaved data message tap discard.
  48. Return the stylus to its holder and remove the pH probe and any save devices.
  49. When you are finished making measurements, rinse the tip of the electrode with distilled water. Slide the cap onto the electrode body, then screw the cap onto the storage bottle.
  50. Note: When the level of storage solution left in the bottle gets low, you can replenish it with pH storage solution. It should NEVER be stored in distilled water!