BUFFERED REACTIONS
Objective: To observe the buffering effect of an acetic acid/sodium acetate buffer
Procedure
1. Put the number of drops of each solution into the circled areas on the sheet protector.
2. Use your stirring rod to mix the solutions carefully.
3. Determine the pH of each solution by touching the end of the stirring rod to a piece of pH
paper and then comparing the resulting color to the color grid.
4. Record the pH for each solution in the data table below.
Data
pH of pure solution
pH with 2 drops
HCl
pH with 2 drops
NaOH
10 drops H2O
10 drops Buffer
Questions
1. What are the formulas for acetic acid and sodium acetate?
2. Write the Ka expression for acetic acid
3. If the Ka for acetic acid is 1.8 x 10-5, what is its pKa? How does this value compare with the
pH of the pure buffer solution?
4. By how many pH units did the pH change when you added the HCl or the NaOH to the
water?
5. By how many pH units did the pH change when you added the HCl or the NaOH to the
buffer?
6. Why is your answer to #5 smaller than #4?
MICROSCALE TITRATION
Objective: To determine the molarity of acetic acid in vinegar
Procedure
1. Put 2 drops of vinegar in one of the circles on the sheet protector (each lab partner will do a
titration and will use a different circle).
2. Add one drop of the indicator, phenolphthalein to the circle and mix.
3. Count the number of drops of 0.1 M NaOH solution that it takes to turn the solution a
permanent light pink. (Use your stirring rod to mix the solutions carefully between drops.)
4. Calculate the molarity of the vinegar (M1) using the expression M1V1 = M2V2 where V1 is 2
drops, M2 is 0.10 M NaOH and V2 = the number of drops used for the titration.
Data
Trial#
Name of Titrator
Drops of NaOH
Molarity of Vinegar
1
2
3
4
Average
Questions
5. Why did the solution turn pink at the end?
6. How many drops of the NaOH solution would have been required if its concentration were
0.01 M instead of 0.1 M?
7. Vinegar is supposed to be 5% acetic acid (5 g HC2H3O2/100 mL). What is the expected
molarity?
TITRATION OF VINEGAR
Trial 1
Trial 2
Trial 4
Trial 3
BUFFER ACTIVITY
10 drops H2O
10 drops H2O
2 drops 0.2 M HCl
10 drops H2O
2 drops 0.2 M NaOH
10 drops Buffer
10 drops Buffer
2 drops 0.2 M HCl
10 drops Buffer
2 drops 0.2 M NaOH