1. PREPARATION OF BUFFERS
To make an effective buffer with a specific pH,
choose an acid with a pKa as close as possible to the required pH
Acetic acid has a Ka = 1.8 x 10-5
∴ pKa = 4.74
∴ Acetic acid and the acetate ion can be used to make buffers from pH = 3.74 to pH = 5.74
EX7-1 (of 7)

2. Prepare a pH = 9. 50 buffer by mixing 50
Prepare a pH = 9.50 buffer by mixing 50.0 mL of a weak acid solution with the correct volume of a strong base solution
Chemicals
M HCl (found on the side counter)
1.000 M NaOH
0.40 M NH4Cl
0.40 M NaH2PO4
Solid NaC2H3O2.3H2O
0.1 M NaH2PO4/0.1 M Na2HPO4 buffer
Correct weak acid
Acid Ka
pKa
H3PO4
7.6 x 10-3
2.12
H2PO4-
6.2 x 10-8
7.21
HPO42-
2.1 x 10-13
12.68
HC2H3O2
1.8 x 10-5
4.74
NH4+
5.8 x 10-10
9.23
Closest to 9.50
EX7-2 (of 7)

3. Prepare a pH = 9. 50 buffer by mixing 50
Prepare a pH = 9.50 buffer by mixing 50.0 mL of a weak acid solution with the correct volume of a strong base solution
weak acid :
0.40 M NH4+ from 0.40 M NH4Cl
0.40 mol NH4+
_________________
L solution
x L solution
= mol NH4+
EX7-3 (of 7)

4. Prepare a pH = 9. 50 buffer by mixing 50
Prepare a pH = 9.50 buffer by mixing 50.0 mL of a weak acid solution with the correct volume of a strong base solution
pH = pKa log (nconj base + nstrong base) ___________________________
(nacid – nstrong base)
pH :
9.50
pKa :
9.23
weak conj. base :
0 mol NH3
weak acid :
0.020 mol NH4+
strong base :
x mol OH-
EX7-4 (of 7)

5. Prepare a pH = 9. 50 buffer by mixing 50
Prepare a pH = 9.50 buffer by mixing 50.0 mL of a weak acid solution with the correct volume of a strong base solution
pH = pKa log (nconj base + nstrong base) ___________________________
(nacid – nstrong base)
= log (0 mol + nstrong base) ______________________________
(0.020 mol – nstrong base)
= log nstrong base ______________________________
(0.020 mol – nstrong base)
= nstrong base ______________________________
(0.020 mol – nstrong base)
EX7-5 (of 7)

6. Prepare a pH = 9. 50 buffer by mixing 50
Prepare a pH = 9.50 buffer by mixing 50.0 mL of a weak acid solution with the correct volume of a strong base solution
= nstrong base ______________________________
(0.020 mol – nstrong base)
1.862 (0.020 mol – nstrong base) = nstrong base
– 1.862nstrong base = nstrong base
= nstrong base
= nstrong base
EX7-6 (of 7)

7. Prepare a pH = 9. 50 buffer by mixing 50
Prepare a pH = 9.50 buffer by mixing 50.0 mL of a weak acid solution with the correct volume of a strong base solution
mol NaOH
x L solution
_____________________
1.000 mol NaOH
= L
= 13 mL
EX7-7 (of 7)