Question 14.2: Suppose you need to prepare a buffer with a pH of 9.00. a Wh...
Suppose you need to prepare a buffer with a pH of 9.00.
a Which of the buffer systems in Table 14.1 would you choose?
b What should be the ratio of the concentration of weak acid, HB, to its conjugate
base, B^{-}?
ANALYSIS
Information given: pH (9.00); from part (a): buffer system ({NH_{4}}^{+} /NH_3)
Information implied: K_{a} of {NH_{4}}^{+}
Asked for: [{NH_{4}}^{+} ]/[NH_3]
STRATEGY
1. Find [H^+].
2. Substitute into Equation 14.1 where [HB] is [{NH_{4}}^{+}] and B^- is [NH_{3}].
[H^+] = K_a \times \frac{[HB]}{[B^-]} (14.1)
c What mass in grams of B^- should be added to 245 mL of 0.880 M HB to give a pH of 9.00?
ANALYSIS
Information given: from part (a): buffer system ({NH_{4}}^{+} /NH_3) from part (b): [H^+] (1.0 \times 10^{-9}\>M); {NH_{4}}^{+} /NH_3 (1.8) {NH_4}^{+} solution: V (0.245 L); M (0.880)
Information implied: molar mass of NH_{3}
Asked for: mass of NH_{3} required to prepare a buffer with pH 9.0.
STRATEGY
1. Find mol {NH_4}^{+}.
n = V \times M
2. Since there is only one solution, [{NH_{4}}^{+} ]/[NH_3] = mol {NH_{4}}^{+}/mol\> NH_3 =1.8. Substitute mol {NH_4}^{+} and find mol NH_{3}.
3. Find the mass of NH_{3} required using its molar mass.
| Table 14.1 Buffer Systems at Different pH Values | ||||
| Buffer System | ||||
| Desired pH | Weak Acid | Weak Base | K_a (Weak Acid) | pK_a |
| 4 | Lactic acid (HLac) | Lactate ion (Lac^-) | 1.4 \times 10^{-4} | 3.85 |
| 5 | Acetic acid (HC_{2}H_{3}O_{2}) | Acetate ion ({C_{2}H_{3}O_{2}}^{-}) | 1.8 \times 10^{-5} | 4.74 |
| 6 | Carbonic acid (H_{2}CO_{3}) | Hydrogen carbonate ion ({HCO_{3}}^-) | 4.4 \times 10^{-7} | 6.36 |
| 7 | Dihydrogen phosphate ion ({H_{2}PO_{4}}^{-}) | Hydrogen phosphate ion ({HPO_{4}}^{ 2-}) | 6.2 \times 10^{-8} | 7.21 |
| 8 | Hypochlorous acid (HClO) | Hypochlorite ion (ClO^-) | 2.8 \times 10^{-8} | 7.55 |
| 9 | Ammonium ion ({NH_{4}}^{ +}) | Ammonia (NH_3) | 5.6 \times 10^{-10} | 9.25 |
| 10 | Hydrogen carbonate ion ({HCO_{3}}^{-}) | Carbonate ion ({CO_{3}}^{ 2-}) | 4.7 \times 10^{-11} | 10.32 |
