Acid-Base Chemistry Hands-On Labs, Inc. Version 42-0137-00-03
Review the safety materials and wear goggles when working with chemicals. Read the entire exercise before you begin. Take time to organize the materials you will need and set aside a safe work space in which to complete the exercise.
Experiment Summary:
You will classify solutions as neutral, acidic, or basic using a pH indicator. You will determine if a neutralization reaction has occurred.
1 - Acetic acid CH3COOH, 1.0 M - 3 mL in pipet1 - Ammonium hydroxide NH4OH, 1 M - 3 mL in pipet, also known as aqueous
ammonia2 - Bromothymol blue, 0.04% - 2 mL in pipet1 - Hydrochloric acid HCl, 1.0 M - 3 mL in pipet1 - Limewater, calcium hydroxide Ca(OH)2, saturated - 2 mL in pipet1 - Nitric acid HNO3, 1.0 M - 2 mL in pipet1 - Phosphoric acid H3PO4, 1.0 M - 3 mL in pipet1 - Sodium acetate CH3COONa, 0.5 M - 2 mL in pipet1 - Sodium bicarbonate NaHCO3, 1.0 M - 2 mL in pipet1 - Sodium carbonate Na2CO3, 1.0 M - 3 mL in pipet1 - Sodium dihydrogen phosphate NaH2PO4, 0.5 M - 2 mL in pipet1 - Sodium hydrogen phosphate Na2HPO4, 0.2 M - 2 mL in pipet1 - Sodium hydrogen sulfate NaHSO4, 0.1 M - 2 mL in pipet1 - Sodium hydrogen sulfite NaHSO3, 0.1 M - 2 mL in pipet2 - Sodium hydroxide NaOH, 1.0 M - 3 mL in pipet1 - Sodium phosphate Na3PO4, 0.1 M - 2 mL in pipet1 - Sulfuric acid H2SO4, 1.0 M - 3 mL in pipet
Note: To fully and accurately complete all lab exercises, you will need access to:
1. A computer to upload digital camera images.
2. Basic photo editing software, such as Microsoft Word® or PowerPoint®, to add labels, leader lines, or text to digital photos.
3. Subject-specific textbook or appropriate reference resources from lecture content or other suggested resources.
Note: The packaging and/or materials in this LabPaq kit may differ slightly from that which is listed above. For an exact listing of materials, refer to the Contents List included in your LabPaq kit.
Acids are substances that are characterized by their ability to donate protons, or hydrogen ions (H+), to other substances in a solution. Acids can be strong or weak. Hydrochloric acid (HCl), which may corrode metals and burn human skin, is an example of a strong acid because it ionizes completely in solution. Weak acids do not ionize completely and are found in everyday substances such as chlorogenic acid (C16H18O9), which is found in coffee. See Figure 1A. Acidity is a measure of the amount of dissolved hydrogen ions, [H+], in a solution. The greater number of hydrogen ions in a solution, the more acidic the solution. Strong acids donate hydrogen ions in water, to a much greater extent than weak acids.
Bases are substances that are characterized by their ability to accept protons or produce hydroxide ions (OH-) in an aqueous solution. As for acids, the difference in strength of bases directly relates to the degree in which hydroxide ions are formed by the base in water. Like strong acids, strong bases are also hazardous chemicals. An example of a strong base is sodium hydroxide (NaOH) , which can degrade the proteins on human skin and cause burns. Weak bases are found in many everyday items such as hand soap. See Figure 1B. When feeling the consistency of a base, it feels slippery because it degrades the fatty acids and oils in the skin on contact. However, due to the possibility that an acid or a base could be very dangerous to human tissue, never experiment with an acid or a base by tasting or touching it.
To determine if a substance is an acid or a base, a pH scale may be used. pH is a measure of the concentration of hydrogen ions [H+] in a substance. The equation for pH is:
pH = – log [H+]The equation can be used to calculate the pH for a known concentration of H+ ions. For example, if [H+] = 1.4 x 10-5 M:
pH = -log[H+] pH = -log (1.4 x 10-5) pH = 4.9
The pH of a solution can also be used to determine the concentration of H+ ions present. For example, if the pH = 8.5:
pH = -log[H+] 8.5 = -log[H+] 10-8.5 = [H+] 3.2 x 10-9 M = [H+]
The logarithm for pH is based on a scale of 10. As pH is a negative logarithm, the pH of a substance decreases as the concentration of H+ increases. For example, a pH of 2 is 102 times, or 100 times, more acidic than a pH of 4. Figure 2 shows the relationship between pH and the concentration of [H+] in a solution.
Acids and bases are present in every aspect of our lives. Car batteries contain sulfuric acid (H2SO4), with a pH near zero (0), whereas household bleach (sodium hypochlorite - NaClO) has a pH of about 12.0. Other common acids include: ascorbic acid (H2C6H6O6) - Vitamin C; carbonic acid (H2CO3) - used in soft drinks; and citric acid (H3C6H8O7) found in citrus fruits such as lemons and oranges. Common bases include: aluminum hydroxide (Al(OH)3) - found in antacids and deodorants; sodium bicarbonate (NaHCO3) - baking soda, which can be used for baking, extinguishing fires and as an antacid; and calcium hydroxide (Ca(OH)2) - found in mortar and plaster. Many baking recipes call for lemon juice and baking soda - what happens chemically when these interact, and what is the outcome in baking?
Figure 2. The relationship between pH and the concentration of [H+] in a solution.
On the pH scale, a pH less than 7.0 is acidic, a pH of 7.0 is neutral, and a pH greater than 7.0 is basic. In addition to the concentration of [H+] ions, bases can be measured by the presence of an ion called a hydroxide ion (abbreviated as OH–). For bases, the pH value increases as the amount of hydroxide ions in the solution increases. A neutral substance is a substance that is neither an acid nor a base.
A common way to measure the pH value of a substance is through the use of a pH indicator. A pH indicator changes color at a specific pH or over the course of a pH range. For example, the Bromothymol blue (BTB) indicator is yellow in a pH below 6.0. At a pH of 6.1-7.5, the indicator is a bluish-green, and above 7.6, BTB is blue. Therefore, BTB is a useful indicator to determine whether a substance is an acid (yellow), whether the substance is around neutral (6.1-7.5), or whether the substance is a base (blue). See Figure 3.
Figure 3. Bromothymol blue (BTB) as an indicator. The well on the left shows how BTB reacts to an acid, the well in the middle shows how BTB reacts to a neutral substance, and the well on
the right shows how BTB reacts to a base.
Conjugate Bases and Conjugate Acids
Acids (HA) ionize in water to produce a hydronium ion (H3O+) and a conjugate base (A-). See
Equation 1. Another way to think of a conjugate base is as what remains of the acid after the hydrogen ion has been lost. The conjugate acid is formed when the hydrogen ion is accepted by the base. In the presence of an acid, water will behave as a base (as it is functioning as a hydrogen ion acceptor) and will accept the hydrogen ion donated by the acid to create a hydronium ion (H3O
+). In Equation 1, the conjugate acid is H3O+.
Equation 1. The ionization of an acid in water.
The strength of an acid is dependent on its ability to fully ionize in water. A strong acid fully ionizes into hydronium ions (H3O
+) and a conjugate base (A-). Thus, the equilibrium position for the strong acid lies mostly to the right in Equation 1. For example, hydrochloric acid (HCl) is a strong acid. The stronger the tendency of an acid to ionize in water, the more hydrogen ions [H+] that end up in the solution. Therefore, a strong acid typically has a low pH. See Equation 2.
Equation 2. A strong acid, HCl. Equilibrium lies overwhelmingly to the right.
Weaker acids form solutions with higher pH values than stronger acids of the same concentration. A stronger acid has a weaker conjugate base, with water having a higher affinity for the hydrogen ion than the weaker conjugate base. On the other hand, a weaker acid has a stronger conjugate base, with the stronger conjugate base having a higher affinity for the hydrogen ion than water. Therefore, less of the weaker acid ionizes in water, which results in less [H+] in the solution and a higher pH.
Bases (B), like acids, also ionize in water to form a hydroxide ion (OH-) and a conjugate acid (BH+). See Equation 3. In the presence of a base, water behaves as an acid, because it will donate a hydrogen ion to form the conjugate acid. In Equation 3, the conjugate acid is BH+. The conjugate base is formed from the acid (in this case, water) when the hydrogen ion is donated. In Equation 3, the conjugate base is OH-.
Equation 3. The ionization of a base (B) in water.
Just as for an acid, the strength of a base is dependent on its ability to fully ionize in water. A strong base fully ionizes into hydroxide ions (OH-), while a weak base only partially ionizes. The equilibrium position for the strong base lies mostly to the right in Equation 3. For example, sodium hydroxide (NaOH) is a strong base. The stronger the tendency of a base to ionize in water, the more hydroxide ions (OH-) end up in the solution. Therefore, a strong base has a high pH. See Equation 4.
Equation 4. A strong base, NaOH. Equilibrium lies overwhelmingly to the right.
Weaker bases form solutions with a lower pH than stronger bases at the same concentration. A stronger base has a weaker conjugate acid, with less ability to donate hydrogen ions than water, resulting in a higher concentration of hydroxide ions. Weaker bases have stronger conjugate acids that are more likely to donate hydrogen ions than water is, resulting in lower hydroxide ion concentrations and lower pH.
All acids, weak and strong, produce H+ ions in water, while all bases produce OH- ions in water. Therefore, when an acid and a base are present in the same solution, these ions combine to form water molecules, through a process called neutralization. See Equation 5. Neutralization reactions result in a solution with a pH between that of the original acid and base solutions, depending upon the strength and molarity of those solutions. If equal numbers of moles of a strong acid and strong base are combined, the resulting solution is neutral, or pH 7.0.
Equation 5. Mixing a hydrogen ion and a hydroxide ion creates water, a pH neutral substance.
In the following exercises, you will determine the pH of a variety of chemicals using pH paper and Bromothymol blue as an indicator. In addition, you will mix acids and bases and determine if neutralization has occurred by testing the pH of the solution. Furthermore, the volume of base required to neutralize a series of acids will be computed and tested. A sample calculation for de-termining the amount of 0.5 M NaOH (aq) to neutralize four drops of 0.5 M HCl (aq) is illustrated below.
1. Determine the volume of 4 drops of HCl (aq):
20 drops = 1 mL4 drops HCl x (1 mL/20 drops) = 0.2 mL HCl0.2 mL HCl x (1 L/1,000 mL) = 2 x 10-4 L HCl
2. Calculate the number of moles of 0.5 M HCl present in 2 x 10-4 L HCl:
2 x 10-4 L HCl x (0.5 mol HCl/L) = 1 x 10-4 mol HCl
3. Calculate the number of moles 0.5 M NaOH required to completely react with 0.5 M HCL:
1 x 10-4 mol HCl x (1 mol NaOH/1 mol HCl) = 1 x 10-4 mol NaOH
4. Calculate the volume occupied by 1 x 10-4 mol of 0.5 M NaOH:
1 x 10-4 mol NaOH x (1L/0.5 mol NaOH) = 2 x 10-4 L NaOH 2 x 10-4 L NaOH x (1,000 mL/1 L) = 0.2 mL NaOH
5. Convert the volume of 0.2 mL NaOH to drops:
0.2 mL NaOH x (20 drops/1 mL) = 4 drops NaOH
From this example, 4 drops of 0.5 M NaOH would be required to neutralize 4 drops of 0.5 M HCl.
Exercise 1: Using pH Paper and the Indicator Bromothymol Blue (BTB) In this exercise, you will determine the approximate pH for 16 different chemicals using pH paper and BTB.
Note: It is important to read the entire procedures before beginning the exercise.
Note: View the video, Opening a Chemical Pipette, before continuing the procedures.
Procedure
1. Gather gloves, a sheet of paper, a pencil, 2 disposable pie pans, paper towels, scissors, the 24-well plate, wide-range pH paper, a trash container, and the Acid/Base Chemistry experiment bag.
2. Create labels for the 24-well plate on the sheet of paper as follows:
a. Fold the paper in half.
b. On half of the sheet of paper, place the 24-well plate on the paper and use a pencil to draw around the 24-well plate.
c. Use the pencil to mark where the wells are positioned in each row and column, marking each side of the wells on the paper drawing. See Figure 4.
Figure 4. Mark where each well row and well column starts and ends on the paper below the 24-well plate.
d. Use the edge of the well plate to draw a straight line, connecting the positions of where each well row and well column begins and ends. See Figure 5.
g. Use the pencil to mark where the wells are positioned in each row and column, marking every side of the wells on the paper drawing as you did previously. Refer to Figure 5.
h. Use the edge of the well plate to draw a straight line across the tracing of the well plate, connecting the positions of where each row and column begins and ends as done before. Refer to Figure 5.
• Label the drawing, starting with the first well in the upper left portion of the grid.
• This time, label the paper grid A1-A8 and BA-BH. See Figure 7.
Figure 7. Label the second grid A1-A8 and BA-BH.
3. Turn the paper so the drawing labeled 11A-11H and 12A-12H is facing upward and place the folded paper in a disposable pie pan. Put the 24-well plate on the piece of paper over the markings. See Figure 8.
Figure 8. Place the 24-well plate on the paper over the drawing labeled 11A-11H and 12A-12H.
4. Open the Acid-Base Chemistry bag and arrange the chemicals from the bag in the 24-well plate, with bulbs face down, according to the names of chemicals that correspond to each “ID Number,” as listed in Data Table 1 in your Lab Report Assistant. For example, the pipet labeled “Acetic Acid” (CH3COOH) will go over the well labeled “11A”. See Figure 9.
Figure 9. Place the chemicals, bulb face up, in the well plate according to the corresponding “ID Numbers” in Data Table 1. For example, acetic acid goes in the well that is marked “11A.”
5. Place the 2 pipets containing BTB in 2 unlabeled wells of the 24-well plate, bulb facing downward. See Figure 10 for complete setup.
Figure 10. Complete setup after step 5.
6. Before moving on to the experiment, make an educated guess (acid or base) for each chemical by studying the chemical formula, shown in Data Table 1. Record your guess in the column of Data Table 1 labeled, “Hypothesis: Acid or Base.”
7. Retrieve the wide-range pH paper, and cut each piece of paper in half lengthwise and then again in half widthwise. See Figure 11. Place the cut pieces of pH paper in a pile in a dry location, such as on a sheet of paper, to be used for Exercises 1, 2, and 3.
Figure 11. Cut each sheet of the pH paper in half lengthwise and widthwise. The final pieces are shown on the right, four pieces from the original piece.
Note: View the video, Using and Reading Wide-Range pH Paper, before continuing the procedures.
8. Put on your pair of gloves and safety goggles.
Safety Warning! You will be working with very strong acids and bases. Do not continue this exercise without wearing protective equipment. You should also wear old clothing that will cover your arms and legs.
9. Wet a paper towel and place it on the unused disposable pie pan.
10. Use a pair of scissors to carefully cut off the tip of the first chemical pipet. Cut the pipet over a trash container so the trash container will catch the tip of the pipet.
11. Replace the pipet, bulb facing downward, back into the 24-well plate in its proper location.
12. Use the wet paper towel after cutting the pipet to carefully wipe any residue left on the scissors from the chemical.
13. Repeat steps 10-12, carefully cutting each pipet, one by one, replacing the pipet back in its respective location in the 24-well plate. Wipe off the scissors between each pipet tip cutting.
14. Keep the used paper towel on the pie pan, but also lay a new, dry piece of paper towel flat in the same pie pan.
15. Dispense 2 drops of a chemical into its well and then place the pipet bulb down in a disposable cup.
Note: If an air bubble is caught in the tip of the pipet, expel the first drop onto the wet paper towel located in the pie plate. You want each drop that goes into the well plate to be a full drop.
16. Use one piece of cut pH paper for one well that contains a chemical to determine the approximate pH of the chemical.
a. Dip the paper into the well and pull it out immediately. It is important to quickly remove the pH paper from the well to avoid dissolving the chemicals from the pH paper into the chemicals in the well.
b. Immediately compare the color of the paper to the scale that was provided with the pH paper.
c. Record the pH in Data Table 1 in the column labeled “pH.”
d. Repeat for each chemical, and only use 1 sectioned piece of pH paper for 1 well.
17. Dispense one drop of BTB in each well of columns 11 and 12.
18. Record the color of each well in Data Table 1.
19. Determine whether each chemical was an acid or a base and record your findings in Data Table 1 of your Lab Report Assistant.
Exercise 2: Acid-Base ReactionsIn this exercise, you will perform a series of reactions between a strong acid and a strong base (SA/SB), a weak acid and a strong base (WA/SB), a diprotic acid and a strong base, and a triprotic acid and a strong base. Based on the stoichiometry of the balanced chemical equations, you will calculate the volume of base that must be added to the final well of each reaction row to reach the equivalence point. An assessment of neutralization will be made using both a pH indicator and universal pH strips.
Procedure
1. Gather gloves, safety goggles, a sheet of paper, a pencil, a pie pan, paper towels, scissors, the 24-well plate, pH strips from Exercise 1, a trash container, and the chemical pipets used in Exercise 1.
2. Create labels for the 24-well plate on a sheet of paper as shown in Figure 12. The labels represent the following reactions:
Figure 12. Chart for labeled 24-well plate grid. To save space, the AcOH abbreviation is used for CH3COOH (acetic acid).
3. Place the labeled paper and well plate in a pie pan so that the markings show through the bottom of the wells. See Figure 13.
Figure 13. 24-well plate resting on labeled paper inside a pie pan.
4. Put on our gloves and safety goggles.
5. Retrieve the following opened pipets from Exercise 1: HCl, CH3COOH, H2PO4, and H3SO4.
6. Dispense 4 drops of each acid in each of the labeled wells of the 24-well plate. For example, well A1 contains 4 drops of HCl, while well B1 contains 4 drops of CH3COOH.
Note: If an air bubble is caught in the tip of the pipet, expel the first drop onto the wet paper towel located in the pie plate. You want each drop that goes into the well plate to be a full drop.
7. Dispense 1 drop of BTB indicator in each of the 20 wells containing the acids.
Note: The BTB indicator is yellow in a pH below 6.0.
8. Using the chemical equations from step 4, calculate the number of drops of NaOH required to neutralize each acid. Refer to the Background as needed, recalling that 1 mL = 20 drops.
9. Record the calculated number of NaOH drops for neutralization in Data Table 2 of your Lab Report Assistant.
10. Dispense 1 drop of NaOH into the well A1 containing the 4 drops of HCl and 1 drop of BTB. See Figure 14.
Figure 14. Adding NaOH to a well containing acid and BTB. The solution will be stirred with the toothpick.
11. Use a clean toothpick to stir the solution in the well.
12. Dispose of the toothpick in the trash.
13. Repeat steps 10-13 for the remaining 19 wells containing acid and BTB, dispensing the number of drops of NaOH as listed on the labeled grid below each well and also the calculated drops for neutralization recorded in Data Table 2 for wells A4, B4, C6, and D6.
14. Use a strip of pH paper prepared in Exercise 1 to measure the pH of the solution in well A1.
15. Record the pH of the solution in Data Table 2.
Note: If the color of the pH strip is not an exact match to the pH color chart, estimate the pH value between the closest two colors on the chart.
16. Place the used pH strip on a paper towel beside the well plate.
17. Repeat steps 14-16 for the remaining 19 wells of solution, using a new pH strip for each well.
18. Take a photo of the well plate be used the photo as a reference when answering the questions at the end of this exercise.
19. Resize and insert the image along with your answer for Question A of your Lab Report Assistant.
20. Save the unused pH strips for use in Exercise 3.
21. Properly dispose of the chemical pipets and used pH strips.
22. Clean and return the well plate to the lab kit.
QuestionsA. Visually analyze the wells in the plate. What information can be gleaned from each well
based on the color of the BTB indicator after each reaction has occurred? How do you know if neutralization has occurred? Cite examples from the well plate reactions to support your answer.
B. What factors determine if neutralization occurs during an acid-base reaction?
C. Is the pH at the equivalence point always equal to 7? Explain your answer based on your experimental observations. Include a discussion of equivalence point pH values for strong versus weak acids.
D. In all the chemical reaction performed in Exercise 2, neutralization occurs when the pH is neutral (7) or basic (>7). Did any of the reaction not result in a color change of the BTB indicator from yellow (acid) to blue (base)? Explain why neutralization may not have occurred in one or more reactions.
Exercise 3: Testing the pH of Household ProductsIn this exercise, you will use pH paper to classify five household products.
Safety Warning! Do not mix the household products.
Procedure
1. Put on your safety glasses and gloves, keep them on for the remainder of this experiment.
2. Locate 5 household products to be used for pH classification. Possible substances to test include home cleaning products, soft drinks, or liquid food items.
3. Record the name of 1 chosen household substance in Data Table 3 in your Lab Report Assistant.
4. Predict if the household substance is an acid, base, or neutral. Record the prediction in Data Table 3.
5. Pour a small amount of the household substance in a clean cup.
6. Use 1 cut piece of pH paper to determine the approximate pH of the chemical.
a. Dip the paper into the cup containing the household substance and immediately remove it.
b. Immediately compare the color of the paper to the scale that was provided with the pH paper.
c. Determine the pH of the substance and record the pH in Data Table 3.
7. Use the pH to categorize the substance as an “acid, base, or neutral”. Record the conclusion in Data Table 3.
8. Repeat Steps 3-7 for the remaining 4 household substances. Use only 1 sectioned piece of pH paper for each substance. Use a clean cup for each substance.
9. Clean all items and return to the lab kit for future use.
10. When you are finished uploading photos and data into your Lab Report Assistant, save and send to your instructor.
QuestionsA. What properties did you use to determine whether the substances you tested were acid or
base?
B. Did any of your findings surprise you? Why or why not?
