Designing a buffer solution with a specific ph

Final Report

Exercise 1

CHEM 182 DL1 Using Buffers

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.

1. Describe the purpose of a buf fer.

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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)

2. Write the chemical equations for the neutralization reactions that occurred when HCL and NaOH were added to the buf fer solution.

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.

3. How do the results in Data Tables 1 and 2 support the role of a buf fer?

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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.

4. Describe the buf fer capacity of the acetic acid buf fer solution in relation to the addit ion of both concentrated and dilute acids and bases. Reference the results in Data Tables 1,2,3, and 4 in 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.

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

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Data Table 1: Adding 0.1 M HCl from D1 to A1

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

6. An acetic acid buf fer solution is required to have a pH of 5.27. You have a solution that contains 0.01 mol of acetic acid. What molarity of sodium acetate will you need to add to the solution? T he pK of acetic acid is 4.74. Show all calculations in your answer.a

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Data Table 2: Adding 0.1 M NaOH from D6 to A6

Number of Drops pH of Solution

0

2

4

6

8

10

12

14

16

6

6

6

6

6

6

6

6

6

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Data Table 3: Adding 6 M HCl from Pipet into B1

Number of Drops pH of Solution

0

2

4

6

8

10

12

14

16

6

6

6

6

6

6

6

6

6

Number of Drops pH of Solution

0

2

4

6

8

10

6

6

4

2

2

2

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Data Table 4: Adding 6 M NaOH from Pipet into B6

Data Table 5: Adding 0.1 M HCl from D1 into C1

Data Table 6: Adding 0.1 M NaOH from D6 into C6

Number of Drops pH of Solution

0

2

4

6

8

10

6

6

6

8

10

12

Number of Drops pH of Solution

0

2

4

6

8

10

7

6

4

4

4

4

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Number of Drops pH of Solution

0

2

4

6

8

10

6

8

10

12

12

12