Name _____________________________ Buffer Tutorial Chem 178 Spring 2003 Section __________

If you need help in order to do a problem or activity, you may GOTO the Chemistry Help Center or see your 178 TA. He or she listen and offer advice, but he or she will not tell you if you are right or wrong. Please turn in this tutorial to your TA. The pH of buffer solutions computer simulation to accompany this pre-lab exercise is located via a Chem 178 WebCT link or http://www.chem.iastate.edu/group/Greenbowe/sections/projectfolder/flashfiles/acidbasepH/pHbuffer20.html

 

Activity 1. What is the pH of a 0.10 M HC3H5O3(aq), lactic acid, solution? Ka = 1.4 x 10-4. pH = __________.

 

 

Write a chemical equation that illustrates what happens when pure HC3H5O3 is placed in water.

 

 

What is the pH of a 0.10 M NaC3H5O3(aq), sodium lactate, solution? pH = _________________.

 

Write a chemical equation that illustrates what happens when solid NaC3H5O3 is placed in water.

 

 

 

 

Without using the computer simulation , predict what happens to the pH of 0.10 M HC3H5O3(aq) solution when enough NaC3H5O3(s) is added so that the initial concentration of the NaC3H5O3(aq) is 0.10 M?

The pH of the solution [increase decrease no change] ?

 

Use the computer simulation to check your prediction.

 

Write a chemical equation that illustrates the system that forms when NaC3H5O3(aq) is added to HC3H5O3(aq) .

 

 

Explain why the pH changes when NaC3H5O3(aq) is added.

 

 

 

 

 

 

 

Use the computer simulation to add different concentrations and amounts of NaC3H5O3(aq) and HC3H5O3(aq) to each other. Make at least two other solutions. Record what solutions you mixed and the resultant pH.

 

Activity 2. Use the computer simulation to mix the following solutions. Compare the pH of the solutions.

  1. 100.0 mL 0.500 M HC3H5O3(aq) / 100.0 mL 0.500 M NaC3H5O3(aq) pH = _____________

     

  2. 100.0 mL 0.100 M HC3H5O3(aq) / 100.0 mL 0.500 M NaC3H5O3(aq) pH = _____________

     

  3. 100.0 mL 0.500 M HC3H5O3(aq) / 100.0 mL 0.100 M NaC3H5O3(aq) pH = _____________

 

Explain what causes the difference in pH of the solutions. It will be helpful to write a chemical equation and explain how LeChatelier's Principle applies.

 

 

 

 

 

 

Activity 3. Using the computer simulation, choose two solutions that when mixed will create a 1.0L buffer solution with the designated pH. Record what two solutions you mixed, what the concentrations were, and the amounts.

  1. pH = 4.74 (using acetic acid as one of the components) Ka = 1.8 x 10-5

     

     

     

  2. pH = 5.00 (using acetic acid as one of the components)

     

     

     

  3. pH = 9.25. (using ammonia as one of the components) Kb = 1.8 x 10-5

     

     

     

  4. pH = 8.00 (using ammonia as one of the components).

 

 

 

 

 

Activity 4. Choose one of the solutions you created in Activity 3. Test this solution to see if it is a buffer solution by going to "Part II" of the program. What evidence do you have that you solution is a buffer solution?

 

Write an equation that shows what happens when acid is added to your solution.

 

 

Write an equation that shows what happens when base is added to your solution.

 

 

 

 

Activity 5. Calculate the pH of a solution created by mixing 200.0 mL of 0.400 M acetic acid and 200.0 mL of 1.00 M sodium acetate. Hint: The instant the two solutions are mixed what are the initial concentrations of each? Use the computer simulation to confirm your calculation.