Determining the pH of Common Substances
L M.3
Introduction
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Name:
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Water contains hydrogen ions, H , and hydroxide ions, OH . The relative concentrations of these two ions determine the pH value. Water with a pH of 7 has equal concentrations of these two ions and is considered to be a neutral solution. If a solution is acidic, + + the concentration of H ions exceeds that of the OH ions. In a basic solution, the concentration of OH ions exceeds that of the H ions. On a pH scale of 0 to 14, a value of 0 is the most acidic and 14 the most basic. A change from pH 7 to pH 8 represents a ten-fold increase in the OH concentration.
Purpose The purpose of this activity is to determine the pH of common household substances and to calculate pOH, hydrogen ion concentration, and hydroxide ion concentration.
Caution! Wear safety goggles and a lab apron at all times during this lab. Acids and bases are dangerous chemicals. Alert Mr. Duell immediately if you come into contact with these chemicals.
Equipment §
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Vernier LabQuest2 Interface
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Vernier pH Sensor
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various common substances
Chemicals & Consumables §
distilled water wash bottle
Procedure 1.
Obtain and wear safety goggles and a lab apron.
2.
Turn on your Vernier LabQuest2. This may happen instantly or within 2 minutes – be patient.
3.
Connect a Vernier pH sensor into Channel 1. Write the number of your pH sensor here: _____. The screen will display a red box with a pH reading.
4.
Carefully, unscrew the storage bottle from the end of the pH sensor. Make sure the solution contained in the bottle stays in the bottle. Rinse the tip of the pH sensor with distilled water only.
5.
For each sample to be tested, insert the clean pH sensor tip into the sample and wait a few moments for the pH reading to stabilize. Record the pH reading in the Data/Results Table on Page 2. Rinse the tip of the pH sensor with plenty of distilled water between each test.
6.
When all available samples have been tested, thoroughly rinse the tip of the pH sensor before recapping the sensor in the storage solution bottle. Kindly refill your distilled water wash bottle from the large jug of distilled water in the room.
7.
Return your equipment to its original location. Wash your lab bench and wash your hands.
8.
Before you go today, enter your pH measurements into the Google Form that Mr. Duell has set up.
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Data Analysis
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1.
In the Results section of the Data/Results Table on Page 2, determine if each sample tested was acidic, basic, or neutral. Re-read the Introduction above if you need help.
2.
Using [H ] = 10
3.
Using 14 = pH + pOH, calculate the pOH for each sample.
4.
Using [OH ] = 10
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-pH
, calculate the hydrogen ion concentration for each sample.
-pOH
, calculate the hydroxide ion concentration for each sample.
Duell
Data/Results Table Data Substance
Results Measured pH
Acidic, Basic, or Neutral?
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[H ], M
pOH
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[OH ], M
Post-Activity Questions
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1.
Select an acidic sample from your table. Write its name here: __________________________
2.
Select a basic sample from your table. Write its name here: __________________________
3.
Of the samples you selected in the last two questions, which sample had a higher pH?
4.
Of the samples you selected in the last two questions, which sample had more hydroxide ions present?
5.
Overall, which sample was most acidic? Which was most basic?
6.
Was the distilled water sample you tested a neutral solution? If it was, why? If it wasn’t, why do you think it wasn’t neutral?
7.
Explain why you think drinking soda or orange juice, even though they both have an acidic pH, does not hurt us.
Duell
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8.
The volume of the solutions tested in this lab varied from cup to cup and tray to tray. Would equal solution volumes have an effect on your results? Why or why not?
9.
Heartburn is caused by an influx of stomach acid (HCl) into the esophagus, causing irritation. Some people drink milk to quell heartburn. But, your results probably indicated milk was slightly acidic, right? Explain why you think slightly acidic milk eases the irritation associated with heartburn.
Duell
