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3.
Using the automatic dispenser, add 50.0 mL of ~0.20 M acetic acid (CH3COOH) solution
directly into the 0.20 M CH3COONa solution prepared above. Allow the solution to stand
for 10 minutes to reach room temperature, then determine the pH of this acetic acid-sodium
acetate buffer solution and record this initial value on your Observations Sheet.
Part III.
Study of the Acetic Acid-Acetate Buffer Solution
A.
Determine the Effect of Dilution on a Buffer Solution.
1.
Add 25.0 mL of the buffer solution to a 400 mL beaker. Re-measure and record the pH of
this buffer solution. (The pH should be the same as the initial measured in Step 3 above.)
2.
Add 225 mL of deionized water to the buffer sample. This solution will be a 1/10 dilution
of the original buffer solution. Measure and record the pH of this diluted buffer.
B.
Determine the Effect of Acid and Base additions on the Buffer Solution.
1.
Dispense a fresh 25.0 mL sample of the original buffer solution (i.e. not the diluted
solution) into a clean 50 mL beaker. Re-measure and record the pH of this buffer solution.
(The pH should be the same as the initial measured in Step 3 above.)
To this sample, add 0.20 M HCl from a buret until the pH changes one full pH unit. Do
this by adding the acid
slowly with constant stirring
and monitoring the pH change
continuously. Record the final pH. Record the initial and final buret volumes to determine
the amount of HCl required to decrease the pH by one full pH unit.
2.
After this first full change of one pH unit, continue to add HCl until a second full pH unit
change
has occurred. Record the initial and final buret volumes for this second change as
well as the final pH. (NOTE: This change will require a much smaller volume of HCl, so
add the HCl carefully.)
