|Purpose: To study the relationship between dilutions and the resulting pH.
Time Required: One 50 minute class period
Group Size: 3 students
Prior Knowledge: Powers of ten
- small plastic cups
- 25 mL graduated cylinder
- Distilled water
- Wash bottle
- Measure out 10.0 mL vinegar and place in a plastic cup – mark it cup 1.
- Rinse the graduated cylinder with distilled water, measure 1.0 mL of the vinegar out of cup 1 and add 9.0 mL of distilled water. Pour into a small cup marked cup 2.
- ... Rinse the graduated cylinder with distilled water, measure 1.0 mL of the vinegar out of cup 2 and add 9.0 mL of distilled water. Pour into a small cup marked cup 3.
- Using the pH probe, measure the pH of the solution in cup 1, record.
- Using the wash bottle, rinse the pH probe with distilled water.
- Repeat steps 4 and 5 for Cup 2 and then Cup 3.
- What is the significance in the change of pH from cup 1 through cup 3?
- Does the trend show an INCREASE in acidity or a DECREASE in acidity?
- The pH of pure water is 7. As an acid or base is diluted with water, does the pH move toward, or away, from 7?
- The pH scale is built upon powers of ten. In this activity, enough water was added to dilute an acidic solution tenfold. How many units of pH change occurred?
- The pH rises by 2 units. What is the change in concentration of the acid?
- Predict: A solution has a pH of 1.0. It is diluted by 1000 times the original concentration. What would you expect the pH to be?
- If a base has a pH of 10.0 and it is diluted by tenfold, will the pH rise or fall? By how many units?
- What is the ratio of mL of vinegar in cup 1 compared to mL of vinegar in cup 2?
- What is the ratio of mL of vinegar in cup 1 compared to mL of vinegar in cup 3?
- When the pH changes by one, for example from a pH of 4 to a pH of 3, the strength or concentration of the solution is changing by what factor? When it changes from a pH of 4 to 2, what is the change in concentration?
- If we made a fourth dilution, what would we expect the pH to be?
- How much stronger would the vinegar in cup 1 be compared to the solution in cup 4?