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Experiment 6: Buffers
Reading: Sections 16.1-16.2 in Olmstead and Williams, General Chemistry , 5th^ Ed.
Purpose : The buffering ability and properties under dilution of acetic acid- sodium acetate buffers will be determined. A pH 5 or pH 9 buffer will be prepared using solid sodium acetate or ammonium chloride.
Introduction A buffer is a solution that resists changes in pH upon:
- addition of small amounts of acid or base
- dilution
A buffer is produced by a solution containing comparable amounts of a weak acid and the corresponding conjugate base. For example, a 1:1 mixture of acetic acid (HOAc) and sodium acetate (OAC–) is a commonly used buffer:
HOAc →← H+^ + OAc–^ [HOAc] ≅ [OAc–] (1)
The pH of a buffer solution is calculated from the Ka expression for the acid dissociation:
Ka =
[H+][OAc–] [HOAc] or solving for [H
+] gives: [H+] = Ka^ [HOAc] [OAc–] (2)
The pH is calculated from the previous expression by taking the – log of both sides:
- log [H+] = – log Ka – log
[HOAc] [OAc–] (3)
Given the definitions of the pH = – log [H+] and pKa = – log Ka the expression becomes:
pH = pKa – log
[HOAc] [OAc–] (4)
The ratio of the acid to conjugate base concentrations is conventionally inverted to give:
pH = pKa + log
[OAc–] [HOAc] (5)
which is nothing more than the Ka expression, Eq. 2, rearranged to solve for the pH directly. The response of a buffer to applied stresses is understood using Eq. 5. The addition of small amounts of strong base to the buffer converts acetic acid to acetate:
HOAc + OH–^ → OAc–^ + H 2 O (6)
However, if the moles of added OH–^ are small compared to the initial moles of acetic acid and acetate, the ratio [OAc–]/[HOAc] stays essentially constant and the pH remains relatively unchanged. The addition of small amounts of strong acid converts acetate to acetic acid:
OAc–^ + H+^ → HOAc (7)
However, if the moles of added H+^ are small compared to the initial moles of acetic acid and acetate, the ratio [OAc–]/[HOAc] stays essentially constant and the pH remains relatively unchanged. Since a buffer contains large concentrations of both a weak acid and the conjugate
base, significant amounts of the weak acid are available to react with any added base and significant amounts of the weak base are available to react with any added acid. Consider a solution that contains 0.55 M acetic acid and 0.45 M acetate. The equilibrium position of the mixture is determined by evaluating the equilibrium concentrations:
HOAc →← H+^ + OAc– initital 0.55 M 0 0.45 M change –x +x +x equilibrium 0.55 – x x 0.45 + x (8)
Substitution of the equilibrium concentrations into Eq. 5 gives:
pH = pKa + log
0.45 + x 0.55 – x (9)
For pure 0.5 M acetic acid the degree of dissociation is less than 1%. As a result, x in Eq. 9 is expected to be small. In a solution with comparable initial concentrations of HOAc and OAc–, LeChatelier’s Principle predicts that x is even smaller than in pure acetic acid solutions. The value of x in Eq. 9 is negligible. The concentrations in the ratio are then well approximated by the initial analytical concentrations. In the general case, let the initial analytical concentration of the weak acid be ca and the initial analytical concentration of the conjugate base be cb. Then the pH of a buffer solution is well approximated by the Henderson Hasselbalch equation:
pH = pKa + log
cb ca (10)
Dilution of the buffer solution affects ca and cb by the same factor, so the ratio is unchanged upon dilution. Preparation of a buffer solution is easily accomplished by mixing solutions of the pure weak acid and the pure conjugate base. For example, mixing 110 mL of 0.500 M acetic acid and 90. mL of 0.500 M sodium acetate produces a buffer with 0.275 M acetic acid and 0.225 M acetate, giving cb/ca = 0.82. If only one of the conjugate acid-base pair is available, then addition of strong acid or strong base creates a buffer. For example, the addition of 100.0 mL of 0.500 M NaOH to 100 mL of 1.00 M acetic acid produces acetate ion. Assuming the reaction with the strong base goes to completion:
HOAc + OH–^ → OAc–^ + H 2 O (11) M 1.00 M 0.500 M V 0.100 L 0.100 L Vtot = 0.200 L initial moles 0.100 mol 0.050 mol final moles 0.050 mol 0 0.050 mol
The ratio of the conjugate base and acid is cb/ca = (0.050 mol/Vtot)/(0.050 mol/Vtot) = 1.00. Notice that the total volume cancels, since both species are in the same solution. As a result, the problem can be worked directly in moles. In this laboratory, you will use the acetic acid-sodium acetate buffer to investigate factors that affect a buffer solution. The useful buffering range of the acetic acid-acetate buffer will be determined using buffers with mole ratios of 10:1 and 1:10. The changes in the pH upon a ten- fold dilution of the buffer, upon addition of strong acid, and upon addition of strong base will be determined. Finally, a pH 5 or pH 9 buffer will be prepared using solid sodium acetate or
C. Effect of Addition of Strong Acid or Base
- Measure the pH of approximately 0.001 M NaCl in water. If a 0.001 M solution is not available, add just a small amount of solid NaCl on the end of a scoopla to about 100 mL of water. Note that an approximate pH is fine.
- To 20 mL of 0.001 M NaCl, add 10 mL of 0.10 M HCl. Record the pH. Again, an approximate pH reading is fine.
- To 20 mL of 0.001 M NaCl, add 10 mL of your 0.10 M NaOH. Record the pH.
- To 20 mL of the 1/1 solution of sodium acetate/acetic acid (the original solution- not the dilution), add 10 mL of the 0.10 M HCl. Record the pH.
- To 20 mL of FRESH 1/1 solution of sodium acetate/acetic acid, add 10 mL of ~0.10 M NaOH. Record the pH.
D. Analysis of Results
- For your Part A data, calculate the theoretical pH of each solution you tested. How well do your measured values agree with the expected values?
- From your Part B data, calculate the theoretical pH of each solution you tested. What is unique about a mixture of a weak acid/conjugate base relative to the weak acid or conjugate base alone?
- From your Part C data, calculate the expected pH of each solution you tested. How does a mixture of a weak acid/conjugate base respond to added acid or base relative to 0.001 M NaCl?
Part 3: Creation of Buffers (work individually for Part 3)
The following reagents will be available in the laboratory:
Solids Solutions sodium acetate⋅trihydrate; MM 136.08 g/mol (pKb = 9.25) NaOH (from previous week) ammonium chloride; MM 53.5 g/mol (pKa = 9.25) 0.10 M HCl
Prepare a buffer that will buffer against pH changes at pH 5 or pH 9 (one partner from Part 2 does one pH, another does the other): Possible helpful information:
- A 0.100 M solution has sufficient buffering capacity for the exercise.
- 100 mL volumetric flasks will be available.
- If you have a weak acid, you can conveniently obtain its conjugate base by adding strong base.
- If you have a weak base, you can conveniently obtain its conjugate acid by adding strong acid.
- The pH you chose for your buffer should correspond to the pKa of the reagent you are also choosing to use. After you have created a buffer solution that you believe will act as a good buffer at either pH 5 or pH 9, set up two ~50 mL aliquots in separate beakers. Record the pH of each aliquot, then ask your instructor to add acid and base to your buffer and record the new pH values. If you constructed an effective buffer, then the pH change should be small in each case.
Lab report: Use the report form. To help save time, the Introduction, Theory, and
Procedure sections are written for you. Additional comments in the Introduction, Theory, and Procedure sections are not necessary.
Questions to address in the discussion:
How well do the experimental values compare to the predicted values? Explain possible causes for the differences. To summarize the experiment answer the following questions:
(i). In a buffer, what are the predominant species in solution? (ii). In forming a buffer, why must the acid-conjugate base pair be weak? (iii). What is the position of equilibrium that gives the best ability to resist changes in pH from additions of either strong acid or strong base? What is the position of equilibrium for a 1: buffer? (iv). When choosing appropriate buffer components, what is the importance of the pKa of the acid, compared to the target pH of the buffer?
