of pH = pKa±1.
5. Choose specific acid-base conjugate pairs of suitable for prepare the following beffers（Use Table 4-1 for Ka of acid or Kb of base）：
（a）pH≈；（b）pH≈；（c）[H3O+]≈×10-9 mol·L-1； Solution （a）HAc and Ac- （b）H2PO? and 4NH3
6. Choose the factors that determine the capacity of a buffer from among the following and explain your choices.
(a) Conjugate acid-base pair (b) pH of the buffer (c) Buffer
ranger
(d) Concentration of buffer-component reservoirs
(e) Buffer-component ratio (f) pKa of the acid component
Solution Choose (d) and (e). Buffer capacity depends on both the concentration of the reservoirs and the buffer-component ratio. The more concentrated the components of a buffer, the greater the buffer capacity. When the component ratio is close to one, a buffer is most effective.
7. Would the pH increase or decrease, and would it do so to a larger or small extent, in each of the following cases:
(a) Add 5 drops of mol·L-1 NaOH to 100 mL of mol·L-1 acetate buffer (b) Add 5 drops of mol·L-1 HCl to 100 mL of mol·L-1 acetate buffer (c) Add 5 drops of mol·L-1 NaOH to 100 mL of mol·L-1 HCl
(d) Add 5 drops of mol·L-1 NaOH to distilled water
Solution（a）The pH increases to a small extent;（b）The pH decreases to a small extent;（c）The pH increases to a small extent;（d）The pH increases to a larger extent.
8. Which of the following solutions will show buffer properties (a) 100 mL of mol·L-1 NaC3H5O3 + 150 mL of mol·L-1 HCl (b) 100 mL of mol·L-1 NaC3H5O3 + 50 mL of mol·L-1 HCl (c) 100 mL of mol·L-1 NaC3H5O3+ 50 mL of mol·L-1 NaOH (d) 100 mL of mol·L-1 C3H5O3H + 50 mL of mol·L-1 NaOH
? （c）NH? HPO244and
Solution (b) and (d)
9. A chemist needs a pH buffer. Should she use CH3NH2 and HCl or NH3 and HCl to prepare it Why What is the disadvantage of choosing the other base
Solution The pKa of CH3NH2·HCl is . The pKa of
NH?4is . The pKa of the former
is more close to . A buffer is more effective when the pH is close to pKa. She should choose CH3NH2. The other is not a good choice.
10. An artificial fruit contains g of tartaric acid H2C4H4O6，and g its salt， potassium hydrogen tartrate，per liter. What is the pH of the beverageKa1=×10-3
Solution
20.0g n(KHC4H4O6)188.2g?mol?1pH?pKa1?lg?3.00?lg?3.00.?lg1.45?3.1611.0gn(H2C4H4O6)150.1g?mol?111. What are the [H3O+] and the pH of a benzoate buffer that consists of mol·L-1 C6H5COOH and mol·L-1 C6H5COONaKa of benzoic acid=×10-5。
Solution
c(C6H5COO?)0.28mol?L-1pH?pKa?lg?4.20?lg?4.20?lg0.85?4.13 -1c(C6H5COOH)0.33mol?L[H+]=×10-5 mol·L-1
12. What mass of sodium acetate (NaC2H3O2·3H2O,Mr=·mol-1) and what volume of concentrated acetic acid ·L-1) should be used to prepare 500 mL of a buffer solution at pH= that is mol·L-1 overall
Solution c(CH3COO-) + c(CH3COOH)= mol·L-1
n(CH3COO-) + n(CH3COOH) = mol·L-1 × 500 mL ×
10-3 L·mL-1 = mol
n(CH3COOH) = mol, n(CH3COO-) = mol Mass of sodium acetate = mol × g·mol -1 = g
13. Normal arterial blood has an average pH of . Phosphate ions form one of the key buffering systems in the blood. Find the buffer-component ratio of a KH2PO4/Na2HPO4 solution with this pH pKa2ˊof
Solution
7.40?6.80?lg[H2PO?4]?[HPO24]
= H2PO?4
缓冲溶液习题
