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Dilutions
Purpose: To study the relationship between dilutions and the resulting pH.

Time Required: One 50 minute class period

Group Size: 3 students

Materials Needed:

  • small plastic cups
  • 25 mL graduated cylinder
  • Pipets
  • Distilled water
  • Vinegar
  • Wash bottle
Prior Knowledge: Powers of ten

Procedure

  1. Measure out 10.0 mL vinegar and place in a plastic cup – mark it cup 1.


  2. 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.


  3. ... 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.


  4. Using the pH probe, measure the pH of the solution in cup 1, record.


  5. Using the wash bottle, rinse the pH probe with distilled water.


  6. Repeat steps 4 and 5 for Cup 2 and then Cup 3.
Data Tables:

 pH
Cup 1 
Cup 2 
Cup 3 


 ratio
Ratio of vinegar to total solution in cup 2
(mL)
 
Ratio of vinegar to total solution in cup 3
(mL)
 
Analysis:

  1. What is the significance in the change of pH from cup 1 through cup 3?


  2. Does the trend show an INCREASE in acidity or a DECREASE in acidity?


  3. 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?


  4. 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?


  5. The pH rises by 2 units. What is the change in concentration of the acid?


  6. 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?


  7. 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?


  8. What is the ratio of mL of vinegar in cup 1 compared to mL of vinegar in cup 2?


  9. What is the ratio of mL of vinegar in cup 1 compared to mL of vinegar in cup 3?


  10. 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?


  11. If we made a fourth dilution, what would we expect the pH to be?


  12. How much stronger would the vinegar in cup 1 be compared to the solution in cup 4?





G Waller

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