Introduction
Objectives
Calculate and measure the concentrations of the various species in weak acid-base systems
Calculate the Ka and Kb of weak acids or bases.
Determine the percentage error from the accepted values
Determine the dissociation equations for various salts.
Determine the hydrolysis equations for various salts
Measure the pH of salt solutions.
Purpose: The purpose of this experiment is to study create and stress a system, restore it, and repeat several times, applying Le Chatelier’s Principle to explain what appears to be happening.
(a) Materials
The following materials were used in the laboratory experiment
• pH meter
• 3 sample cups
• Calibration buffers
• Sample test cups
• Buffer solutions (pH of 4 and 7)
• Smelling salts (NH4)2CO3
• Washing soda Na2CO3
• Epsom salt MgSO4
• 4-125 ml Erlenmeyer flasks
• Wash bottle
• vinegar (acetic acid, CH3COOH)
• Seltzer water (carbonic acid, H2CO3)
• Ammonia (ammonium hydroxide, NH4OH)
• Distilled water
• Table salt (NaCl)
• Sharpie permanent marker
• Large drinking glass
• One-fourth teaspoon scoop
(b)Procedures
Preparation: Calibration of the pH meter:
1. It is recommended that this laboratory trilogy be done during a single session, so that it is necessary to calibrate the pH meter only once. You have been given two packets of the calibration buffer solutions, so you can do the lab during two sittings; however, for accuracy and precision’s sake, it is best to do it all at once.
2. Open the package that contains the pH meter and follow the instructions on how to calibrate the meter using the calibration buffer solutions. As you are performing the lab, do not allow the electrodes at the bottom of the pH meter to dry out. While not in direct use, store the pH meter in a small amount of tap water at the bottom of a drinking glass.
Review the video on calibrating the HI98103 pH Meter.
Cautions regarding the pH meter: When you open the cap of the pH meter, you will find that it is stored in a small amount of the buffer seven solution. Please ensure that when you are finished using it, that you store the pH meter with a small amount of the tap water solution in the cap, to keep the electrodes from drying out.
Do not throw out buffer solution!! You will want to fold over and seal the solution for Lab 6 and 7.
Experimentation: (Remember goggles and appropriate safety precautions!)
1. Using the sample cups, place a small amount of the three liquids (vinegar, seltzer water, and ammonia) into three separate, labeled sample cups.
2. Using the calibrated pH meter, determine the pH of the three liquids, making sure to rinse off the pH meter with tap water after each measurement is taken. Record the value for the pH of each substance in your data table.
pH level
Vinegar (acetic acid, CH3COOH) 2.21
Seltzer water (carbonic acid, H2CO3) 3.98
Ammonia (ammonium hydroxide, NH4OH) 10.57
Part II:
Preparation
Using the Sharpie permanent marker, label each of the four (4) sample cups with the chemical formula for each of the four (4) reagents (NaCl, Na2CO3, (NH4)2CO3, and MgSO4).
Using a pencil, label the four (4) Erlenmeyer flasks on the white part of the etched glass for each flask with the same four (4) reagent labels (NaCl, Na2CO3, (NH4)2CO3, and MgSO4).
Fill the wash bottle with distilled water.
Fill the drinking glass half-full with the distilled water.
Making solutions
1. Using the one-fourth teaspoon scoop, place a level scoop of the washing soda (Na2CO3) into the labeled Erlenmeyer flask, and add 100 ml of distilled water. Swirl until the solid is completely dissolved.
2. Repeat Step 1 for the other three (3) reagents. (NaCl, (NH4 )2CO3, and MgSO4 )
pH level
Washing soda (Na2CO3) 10.73
Table salt NaCl 7.68
Smelling salts (NH4 )2CO3 7.40
Epsom salt MgSO4 8.83
Calibrating pH meter
1. Using instructions included in the box with the pH meter, calibrate the pH meter for both acid calibration and alkaline calibration.
2. Place the calibrated pH meter in the drinking glass with the distilled water to keep it constantly wet while you perform the experiment.
Procedure:
1. Fill each of the labeled sample cups half-full of the corresponding reagent.
2. Using the pH meter, determine the pH of each of the test solutions. Be sure to rinse the electrodes of the meter over your sink, using the distilled water in the wash bottle in between each reading.
3. You may need to recalibrate the pH meter in between readings if the pH seems to drift more than a single pH unit.
4. Record the pH values of each solution.
Part I
Calculations:
1. Calculate the hydrogen ion concentration, [H+ ], for the two weak acids (pH=-log[H+ ], or [H+ ]=antilog (-pH). If you have difficulty finding or using the antilog function on your calculator, simply use this: [H+ ]=10-pH . Record these in your data table.
2. Calculate the hydroxide ion concentration, [OH- ], for the weak base using this formula: pOH=14-pH, then [OH- ]=antilog (-pOH) or [OH- ]=10-pOH. Record this in your data table.
3. Record [A- ] or [BH+ ] in your data table. Refer to the information provided in the introductory section of this lab.
4. The concentration of carbonic acid, which is the seltzer water, can be found in a handbook of chemistry and physics and is 0.0341 M (at STP). Record this in your data table.
5. You will need to calculate the molar concentrations of the vinegar as well as ammonia. Both are industry standard 5.00% by mass solutions in water. This means 5.00 g of solute/ 100.00 g of solution. Assume that the solution is mostly water (which it is) and thus has a density of 1.00 g H2O/1.00 ml H2O. Now use this information plus the molar masses of the vinegar (acetic acid) and ammonia (ammonium hydroxide) to change the 5.00 g solute/100.00 g solution to M, which is mol solute/L solution (H2O). Record these concentrations in your data table.
6. Calculate the Ka or Kb using the formula given in the introduction. Record these in your data table.
7. Calculate the pKa or pKb (pKa/b =-log Ka/b) and record these in your data table.
Discussion Questions:
1. Find the Ka/b values in Appendix D tables 1 (Ka1 only) and 2 and calculate the pKa/b values.
2. Calculate your percentage error.
3. Explain mathematically and chemically why you cannot measure or calculate a pKa or pKb for a strong acid or strong base.
Part II
Discussion
1. Salts can be formed by the reaction of an acid with a base. For each of the four salts you tested, write the reaction of an acid with a base to form that salt. Indicate whether the acid and base are strong or weak.
2. Write the dissociation equations for each of the four salts (into ions- you should use the subscript (aq) for them).
3. Write the hydrolysis equations for each of the four (4) salts (see intro for examples).
4. According to your data, indicate which salts appear to be neutral, acidic, and alkaline.
5. Explain why each salt appears to be neutral, acidic, or alkaline.
6. List an example other than the salts used in this lab for each of the following:
a. a neutral salt (formed by the reaction of a strong acid with a strong base)
b. a neutral salt (formed by the reaction of a weak acid with a weak base)
c. an acidic salt
d. an alkaline salt
7. In the lab, which of the salts could be used to make a buffer and why?
Lab 5
Ka, Kb, and Salt Hydrolysis
Introduction: Connecting Your Learning
The majority of acids and bases are actually weak acids and bases, which mean that in aqueous solution, most of the acid or base molecules exist in molecular form. They do not ionize to a large degree to form the hydrogen ion (H+(aq)) or hydroxide ion (OH-(aq)). These are especially common in living organisms such as the organic acids called amino acids, fatty acids, and nucleic acids. Palmitic acid (C15H31COOH) is a “saturated” (full of hydrogen’s with no C-C double bonds) fatty acid common in palm oil. Linoleic acid (CH3(CH2CH=CH)3(CH3)7COOH) is an “unsaturated” (has C-C double bonds which means fewer hydrogen’s) fatty acid found in linseed oil. Adrenaline and norepinephrine are weak bases that are also nervous system hormones and neurotransmitters. The artificial sweetener aspartame consists of two amino acids: aspartic acid and phenylalanine.
Weak acids (HA) can be represented as ionizing or dissociating according to the following equilibrium equation:
Ka is the acid dissociation constant, which is just a special designation for Keq, the general equilibrium constant. [HA] is the concentration of the undissociated weak acid, [H+] is the hydrogen ion concentration, and [A-] is the concentration of the anion (“conjugate base” in the Brønsted-Lowry concept). For weak bases (B), a similar representation can be used as follows:
Kb is the base dissociation constant. [B] is the concentration of the undissociated weak base, [BH+] is the concentration of the cation from the dissociated weak base (“conjugate acid” in the Brønsted-Lowry concept), and [OH-] is the hydroxide concentration, leaving out [H2O], or the concentration of water since the concentration of a pure liquid does not change appreciably.
If the weak acid or base dissociates very little (i.e., has a very small Ka/Kb, about 10-4), then the following approximations can be made:
[HA] or [B] = given concentration; that is, if it is given as 1.00 M, then [HA] or [B] = 1.00 M (in other words, the amount of HA or B that actually dissociates into ions is small enough to be beyond the number of significant figures being used).
Also, [H+]=[A-] or [BH+]=[OH-] since for every molecule that dissociates, it must dissociate into those ions.
Thus, if you know the molarity of the weak acid or weak base and can measure its pH, you can calculate its Ka or Kb.
Scattered throughout some of the arid regions in the western part of the United States are dry lakebeds often appearing on road maps as dotted blue line regions. They are typically labeled as “alkali lakes” or salt flats. Typically formed by runoff from surrounding mountains over eons of time, these alkali lakes tend to dry up. When the wind blows this alkaline salt deposit as dust in the air, it can be very corrosive since the salts tend to hydrolyze water and increase the hydroxide ion concentration. Borax (Na2Br4O7), a widely used water softener, is one of those salts that accumulate in this manner and is mined in those areas. It hydrolyzes water in the following manner:
(As a water softener, the borate ion combines with Ca2+(aq) present, effectively removing this “harder” ion from the water.) The Na+(aq) does not combine with the OH-(aq) since NaOH is a strong base that dissociates rather than stays combined, so sodium borate increases the pH in a solution; namely, it is an alkaline salt.
Mining tailings (the materials left after the main ores are extracted from the mine) frequently contain minerals that had been leached over eons of time deep into the earth but now are exposed to weathering and erosion. Many of these exposed tailings contain transition metal salts that are chemically the opposite of the alkaline salts mentioned above; that is, they are acidic salts. If you have ever seen some of the containment ponds or leach pools leftover from a mining operation, you probably noticed the distinctive color of the water, sometimes a very murky orange. Iron(III) oxide (Fe2O3), although not very soluble, will slowly dissolve in acidic solutions. Since rainfall is naturally slightly acidic (you will cover this more in Chapter 18), Fe3+(aq) form in these pools, and then hydrolyze water in the following manner:
This increases the [H+], or lowers the pH.
Thus salt solutions can have a range of pH values, depending upon the nature of the salt.
In the following lab, you will 1) measure the pH of weak acids and bases of known concentrations to calculate their Ka or Kb, and 2) determine your percentage error in Part I. In Part II, you will also 1) predict the relative pH of various salt solutions, and 2) explain the results.
Resources and Assignments
Multimedia Resources None
Required Assignments Lab 5 Report
Materials (Lab Kit) Part I
• pH meter
• 3 sample cups
• Calibration buffers
Part II
• pH meter
• Sample test cups
• Buffer solutions (pH of 4 and 7)
• Smelling salts (NH4)2CO3
• Washing soda Na2CO3
• Epsom salt MgSO4
• 4-125 ml Erlenmeyer flasks
• Wash bottle
Materials (Student supplied) Part I
• vinegar (acetic acid, CH3COOH)
• Seltzer water (carbonic acid, H2CO3)
• Ammonia (ammonium hydroxide, NH4OH)
Part II
• Distilled water
• Table salt (NaCl)
• Sharpie permanent marker
• Large drinking glass
• One-fourth teaspoon scoop
Focusing Your Learning
Lab Objectives
By the end of this lesson, you should be able to:
1. Calculate and measure the concentrations of the various species in weak acid-base systems.
2. Calculate the Ka and Kb of weak acids or bases.
3. Determine the percentage error from the accepted values.
4. Determine the dissociation equations for various salts.
5. Determine the hydrolysis equations for various salts.
6. Measure the pH of salt solutions.
Procedures
Part I
Preparation: Calibration of the pH meter:
1. It is recommended that this laboratory trilogy be done during a single session, so that it is necessary to calibrate the pH meter only once. You have been given two packets of the calibration buffer solutions, so you can do the lab during two sittings; however, for accuracy and precision’s sake, it is best to do it all at once.
2. Open the package that contains the pH meter and follow the instructions on how to calibrate the meter using the calibration buffer solutions. As you are performing the lab, do not allow the electrodes at the bottom of the pH meter to dry out. While not in direct use, store the pH meter in a small amount of tap water at the bottom of a drinking glass.
Review the video on calibrating the HI98103 pH Meter.
Cautions regarding the pH meter: When you open the cap of the pH meter, you will find that it is stored in a small amount of the buffer seven solution. Please ensure that when you are finished using it, that you store the pH meter with a small amount of the tap water solution in the cap, to keep the electrodes from drying out.
Do not throw out buffer solution!! You will want to fold over and seal the solution for Lab 6 and 7.
Experimentation: (Remember goggles and appropriate safety precautions!)
1. Using the sample cups, place a small amount of the three liquids (vinegar, seltzer water, and ammonia) into three separate, labeled sample cups.
2. Using the calibrated pH meter, determine the pH of the three liquids, making sure to rinse off the pH meter with tap water after each measurement is taken. Record the value for the pH of each substance in your data table.
Part II:
Preparation
Using the Sharpie permanent marker, label each of the four (4) sample cups with the chemical formula for each of the four (4) reagents (NaCl, Na2CO3, (NH4)2CO3, and MgSO4).
Using a pencil, label the four (4) Erlenmeyer flasks on the white part of the etched glass for each flask with the same four (4) reagent labels (NaCl, Na2CO3, (NH4)2CO3, and MgSO4).
Fill the wash bottle with distilled water.
Fill the drinking glass half-full with the distilled water.
Making solutions
1. Using the one-fourth teaspoon scoop, place a level scoop of the washing soda (Na2CO3) into the labeled Erlenmeyer flask, and add 100 ml of distilled water. Swirl until the solid is completely dissolved.
2. Repeat Step 1 for the other three (3) reagents. (NaCl, (NH4 )2CO3, and MgSO4 )
Calibrating pH meter
1. Using instructions included in the box with the pH meter, calibrate the pH meter for both acid calibration and alkaline calibration.
2. Place the calibrated pH meter in the drinking glass with the distilled water to keep it constantly wet while you perform the experiment.
Procedure:
1. Fill each of the labeled sample cups half-full of the corresponding reagent.
2. Using the pH meter, determine the pH of each of the test solutions. Be sure to rinse the electrodes of the meter over your sink, using the distilled water in the wash bottle in between each reading.
3. You may need to recalibrate the pH meter in between readings if the pH seems to drift more than a single pH unit.
4. Record the pH values of each solution.
Assessing Your Learning
Part I
Calculations:
1. Calculate the hydrogen ion concentration, [H+ ], for the two weak acids (pH=-log[H+ ], or [H+ ]=antilog (-pH). If you have difficulty finding or using the antilog function on your calculator, simply use this: [H+ ]=10-pH . Record these in your data table.
2. Calculate the hydroxide ion concentration, [OH- ], for the weak base using this formula: pOH=14-pH, then [OH- ]=antilog (-pOH) or [OH- ]=10-pOH. Record this in your data table.
3. Record [A- ] or [BH+ ] in your data table. Refer to the information provided in the introductory section of this lab.
4. The concentration of carbonic acid, which is the seltzer water, can be found in a handbook of chemistry and physics and is 0.0341 M (at STP). Record this in your data table.
5. You will need to calculate the molar concentrations of the vinegar as well as ammonia. Both are industry standard 5.00% by mass solutions in water. This means 5.00 g of solute/ 100.00 g of solution. Assume that the solution is mostly water (which it is) and thus has a density of 1.00 g H2O/1.00 ml H2O. Now use this information plus the molar masses of the vinegar (acetic acid) and ammonia (ammonium hydroxide) to change the 5.00 g solute/100.00 g solution to M, which is mol solute/L solution (H2O). Record these concentrations in your data table.
6. Calculate the Ka or Kb using the formula given in the introduction. Record these in your data table.
7. Calculate the pKa or pKb (pKa/b =-log Ka/b) and record these in your data table.
Discussion Questions:
1. Find the Ka/b values in Appendix D tables 1 (Ka1 only) and 2 and calculate the pKa/b values.
2. Calculate your percentage error.
3. Explain mathematically and chemically why you cannot measure or calculate a pKa or pKb for a strong acid or strong base.
Part II
Discussion
1. Salts can be formed by the reaction of an acid with a base. For each of the four salts you tested, write the reaction of an acid with a base to form that salt. Indicate whether the acid and base are strong or weak.
2. Write the dissociation equations for each of the four salts (into ions- you should use the subscript (aq) for them).
3. Write the hydrolysis equations for each of the four (4) salts (see intro for examples).
4. According to your data, indicate which salts appear to be neutral, acidic, and alkaline.
5. Explain why each salt appears to be neutral, acidic, or alkaline.
6. List an example other than the salts used in this lab for each of the following:
a. a neutral salt (formed by the reaction of a strong acid with a strong base)
b. a neutral salt (formed by the reaction of a weak acid with a weak base)
c. an acidic salt
d. an alkaline salt
7. In the lab, which of the salts could be used to make a buffer and why?
Lab Report
Make sure to complete a full lab report and submit it via email by the due date.
Submission
Important information: Please follow the procedure below in the completion of your assignments.
Compose your responses to the questions in a word processing program. Run spell check. Review your work to make sure that you have completely answered all questions.
To upload your Lab from this assessment you must first click on the “Browse” button. Now click on the “Browse” button once the File upload window pops up. There should now be another pop up window, which will allow you to choose the location of the file you’re uploading. Select your file and then press “Open”. You will now be taken back to the File upload window; once here click on the “Submit” button and then click on the “Close” button. Once you’re back to your assessment window you can submit your assessment.
When you have finished, you will need to close the browser window to return to your course.
Select the following link to upload: Lab 5.
TAKE ADVANTAGE OF OUR PROMOTIONAL DISCOUNT DISPLAYED ON THE WEBSITE AND GET A DISCOUNT FOR YOUR PAPER NOW!