pH
Developing the Concept of pH
Even though you may not realize it you probably interact with acids and bases every day.
Drinking a soda, eating a pickle, washing your hands all involve the substances we refer to as
acids and bases. There are many ways to define acids and bases, but the most common way to
describe them is with the pH scale. Acidic solutions are those that have a pH less than 7 and
have higher concentrations of hydrogen ions than hydroxide ions (OH−). Conversely, solutions
with a pH greater than 7 have lower concentrations of hydrogen ions and higher concentrations
of hydroxide ions. A solution that has an equal amount of both hydrogen and hydroxide ions is
neutral. A neutral solution has a pH of 7.0.
PURPOSE
In this lab you will explore some common household substances for their acidic and basic nature
and predict their relative pHs. Ultimately, you will mix some acidic and basic solutions together
to see what effect this has on pH.
MATERIALS
safety goggles
lab apron
microchem plate, 24 deep wells
2 1mL syringes
8 pipettes
universal indicator with pH scale
pH meter
8 beakers or plastic cups for solutions
1 M HCl solution
paper towels
Solutions from household substances:
dish soap solution
baking soda solution
vinegar
colorless carbonated drink
lemonade or limeade
distilled water
ammonia
SAFETY ALERT
Safety goggles should be worn at all times when working with acids or bases.
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PROCEDURE
PART I
1. Put on your goggles and apron.
2. Obtain the various solutions and materials for the lab.
3. In the first column of Data Table 1, rank the solutions in order from what you predict will be
the most acidic solutions to the most basic. Make qualitative observations of the various
solutions. Do not touch or taste the solutions. Record your observations in Data Table 1
on your student answer page.
4. Place a white sheet of paper under the microchem plate. Label the wells with the various
solutions by writing the solutions’ names on the paper next to wells that they will be placed
in.
5. Using a different pipette for each solution, add 10 to 15 drops of the solutions into the
appropriate wells.
6. Add a drop of universal indicator to each well with the last pipette. Record the color of the
resulting solution in Data Table 1. Use the scale that comes with the Universal indicator to
determine the pH of the solutions. Record this value as well.
7. Complete the analysis questions for Part I on you student answer page. Then, fill out the
Venn diagram to compare and contrast acids and bases. Be sure to not only note
observations that you have made during this lab, but also observations you may have from
previous experience.
8. After completing your Venn diagram wait for your teacher to lead a class discussion before
continuing with the remainder of the activity.
9. When you teacher instructs, obtain an additional beaker or plastic cup to act as a reaction
vessel. Choose 3 pairs of acid + base from your table to combine.
10. Record the identity of the acids and bases you chose in Data Table 2 on your student answer
page. Predict what the new pH might be when equal amounts of the two solutions are
combined. Record your predictions in Data Table 2.
11. Once your predictions have been made, add an equal number of drops of both the acid and
the base to the reaction vessel. Test the pH of the solution with universal indicator and
record the pH in Data Table 2.
12. Repeat the process with the remaining two combinations.
13. Clean your cups and well plate as your teacher instructs.
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PART II
1. Obtain two clean beakers or cups. Fill one beaker with 50 mL of distilled water and label it
dH2O. Fill the second beaker with 100 mL of tap water and label it RINSE WATER. This
beaker will be used for rinsing the syringes and the pH probe. Label one syringe ACID and
the other syringe WATER.
2. Your teacher will put 1 mL of the HCl solution into well #1 on your well plate. Determine
the pH of the solution with the pH probe. Record this value in the Source pH column in Data
Table 3 on your student answer page.
3. Using your acid pipette, take 0.1 mL of the solution from well #1 and place it into well #2.
Rinse the syringe thoroughly with the rinse water after you have finished this step.
4. Add 0.9 mL of distilled water to well # 2. This will make a total of 1 mL of liquid in the
well.
5. Predict the pH of this diluted solution in Data Table 3. Record the dilution factor as 1/10
since there is 0.1 mL of acid per 1.0 mL total.
6. Test the actual pH with a pH probe. Record this value in Data Table 3. Do not forget to
rinse the pH probe!
7. To create the solution for well #3, take 0.1 mL of the solution created in well #2 and add it to
well #3 along with 0.9 mL of distilled water. Remember, the source pH will be the same as
the actual pH of the previous well.
8. Use the pH probe to measure the actual pH of well #3.
9. Predict the pH for well #4 before you proceed. Once you have made your prediction create
well #4 using the same process for the other wells, and test your prediction.
10. Clean up your area as directed by the teacher and answer the conclusion questions.
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pH
Puzzling with pH
DATA AND OBSERVATIONS
PART I
Data Table 1: Initial Observations
Solutions
(in order of
predicted pH)
Observations
Color with
Universal
Indicator
pH
1. How close were you with your predictions of the pHs of the acids and bases? What are some
properties that you can use to determine if a solution is acidic or basic?
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2. Complete the Venn diagram on acids and bases. Be sure to include properties of acids and
bases that you have determined so far and uses that each is often used for.
Data Table 2: Acid + Base
Acid and Base Selected
Predicted pH
Actual pH
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PART II
Data Table 3: Dilutions
Well
#
1
Source pH
Predicted pH
-------------------
Dilution
Factor
Actual pH
-----------------
For this well the
pH will be the
same as the
source pH
2
3
4
ANALYSIS
PART I
1. Consider ammonia. How did the pH change when vinegar (which contains acetic acid) was
added? Explain what you think occurred to cause the change.
2. How did the actual pH compare to the predicted pH? What were possible sources of error?
PART II
1. What do you predict the units of H+ will be in well number 4?
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CONCLUSION QUESTIONS
PART I
1. What are the characteristics of an acidic solution?
2. How are the characteristics of a basic solution different than an acid?
3. How would the properties of a solution with a pH of 6.0 compare to another solution with a
pH of 3.0?
4. You are given a pH 6.0 acidic solution and a pH 9.0 basic solution. Describe how you might
combine the solutions to achieve a final pH of 7.0.
PART II
1. Temperature and pH are both described on scales because each incremental increase is
consistently sized. Contrast what you have learned about the pH scale to what you already
know about the Celsius temperature scale.
2. Well number 4 will be _________ times less concentrated than well number 1.
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3. Would it be possible to continue diluting the acid in this way to reach a pH of 10? Why or
why not?
4. During the dilution process your partner informs you that he used the same pipette for
transferring the solutions as he did for adding distilled water. He also neglected to rinse the
pipette. Would this error result in pH values that were higher, lower, or exactly the same as
what was expected? Explain your answer.
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