Purpose of a buffer system

Heather Gilbertson

Lab 12: Using Buffers

Data Table 1. Adding 0.1M HCl from D1 into A1 sodium acetate buffer solution.

Drop Number

pH of Solution

0 drops added

1

6.0

2

6.0

3

6.0

4

6.0

5

6.0

6

6.0

7

6.0

8

6.0

9

6.0

10

6.0

11

6.0

12

6.0

13

6.0

14

6.0

15

6.0

Data Table 2. Adding 0.1M NaOH from D6 into A6 sodium acetate buffer solution.

Drop Number

pH of Solution

0 drops added

1

6.0

2

6.0

3

6.0

4

6.0

5

6.0

6

6.0

7

6.0

8

6.0

9

6.0

10

6.0

11

6.0

12

6.0

13

6.0

14

6.0

15

6.0

Data Table 3. Adding 6M HCl from Pipet into B1 sodium acetate buffer solution.

Drop Number

pH of Solution

0 drops added

1

6.0

2

6.0

3

5.0

4

4.0

5

3.0

Data Table 4. Adding 6M NaOH from Pipet into B6 sodium acetate buffer solution.

Drop Number

pH of Solution

0 drops added

1

6.0

2

6.0

3

7.0

4

8.0

5

8.0

Data Table 5. Adding 0.1M HCl from D1 into C1 containing distilled water.

Drop Number

pH of Solution

1

7.0

2

6.0

3

5.0

4

4.0

5

4.0

Data Table 6. Adding 0.1M NaOH from D6 into C6 containing distilled water.

Drop Number

pH of Solution

1

8.0

2

8.0

3

9.0

4

10.0

5

11.0

Questions

1. Describe the purpose of a buffer.

The purpose of a buffer is to maintain a nearly constant pH by minimizing the change in pH, when a little bit of acid or base is added to the solution.

2. Write the acid base neutralization reaction between the buffer and the added HCl. (You don’t need to include the spectator ions.) Write the acid-base neutralization reaction between the buffer and the added NaOH.

Buffer Solution:

CH3COOH(aq)+NaOH(aq)CH3CooNa(aq)+H2O(l)

The buffer solution contains acetic acid and its weak acid and sodium acetate as its conjugate base. If we add strong HCI to the buffer solution above, the sodium acetate would react with HCI as in the chemical equation below:

HCI(aq)+CH3COONa(s)CH3COOH(aq)+NaCl(s)

If we add sodium hydroxide to the buffer solution the acetic acid in the buffer would react as in the following chemical e=reaction equation:

NaOH(aq)+CH3COOH(aq)CH3COONa(s)+H2O(l)

3. Explain how the data in tables 1 and 2 supports the role of a buffer.

In the presence of a buffer, the addition of 0.1M HCl or 0.1 M NaOH resulted in no change in pH. The solution was capable of maintaining a nearly constant pH when the smaller/ dilute concentrations of acid or base were added.

4. Describe the buffer capacity of the acetic acid buffer solution in relation to the addition of both concentrated and dilute acids and bases. Reference the results in Data Tables 1, 2, 3 and 4 in your answer.

In the presence of the acetic acid buffer solution, addition of dilute acid and dilute base did not affect the pH of the solution, as shown in tables 1 and 2. However, addition of a higher concentration of the strong acid and base altered the pH level significantly. The concentrated solutions of either the acid or the base has a significant effect in the pH of acetic acid buffer solution while the diluted acids and bases had no effect on the pH showing the buffering capacity of the acetic acid buffer solution could withstand only small amounts of the dilute acids and bases.

5. Did distilled water act as a buffer in the experiment? Use the results in Data Tables 5 and 6 to support your answer.

Distilled water did not act as a buffer because it was not able to withstand or buffer the solution upon addition of an acid or base.

6. An acetic acid buffer solution is required to have a pH of 5.27. You have a solution that contains 0.01 mol of acetic acid. How many moles of sodium acetate will you need to add to the solution? The pKa of acetic acid is 4.74. Show all calculations in your answer. pH = pKa + log [A- ]/[HA]

PH = PKa + log[CH3COONa]/[CH3COOH]

5.27 = 4.74 + log[CH3COONa]/1

log[CH3COONa] = 5.27-4.74

log[CH3COONa] = 0.53

no of moles of CH3COONa = 100.53 = 3.3884moles