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)

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 0.1000 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)

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 0.0500 L solution = 0.020 mol NH4+ EX7-3 (of 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 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)

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) 9.50 = 9.23 + log (0 mol + nstrong base) ______________________________ (0.020 mol – nstrong base) 0.27 = log nstrong base ______________________________ (0.020 mol – nstrong base) 1.862 = nstrong base ______________________________ (0.020 mol – nstrong base) EX7-5 (of 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 1.862 = nstrong base ______________________________ (0.020 mol – nstrong base) 1.862 (0.020 mol – nstrong base) = nstrong base 0.03724 – 1.862nstrong base = nstrong base 0.03724 = 2.862nstrong base 0.01301 = nstrong base EX7-6 (of 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 0.01301 mol NaOH x L solution _____________________ 1.000 mol NaOH = 0.013 L = 13 mL EX7-7 (of 7)