第14章酸塩基平衡

Key Concepts and Summary重要な概念とまとめ

Solutions that contain appreciable amounts of a weak conjugate acid-base pair are called buffers. A buffered solution will experience only slight changes in pH when small amounts of acid or base are added. Addition of large amounts of acid or base can exceed the buffer capacity, consuming most of one conjugate partner and preventing further buffering action.

弱い共役酸塩基対を十分な量だけ含む溶液を、緩衝液という。緩衝液に少量の酸や塩基を加えても、pHの変化はわずかである。多量の酸や塩基を加えると緩衝容量を超え、共役対の一方の大部分が消費されて、それ以上の緩衝作用が働かなくなる場合がある。

Key Equations重要な式

pK_a = −log K_a

pK_b = −log K_b

pH = p K_{a} + log \frac{[A^{−}]}{[HA]}

Exercises 77–91練習問題 77–91

Exercise 77問題 77

Explain why a buffer can be prepared from a mixture of NH₄Cl and NaOH but not from NH₃ and NaOH.

NH₄ClとNaOHの混合物からは緩衝液を作れるが、NH₃とNaOHからは作れない理由を説明しなさい。

Exercise 78問題 78

Explain why the pH does not change significantly when a small amount of an acid or a base is added to a solution that contains equal amounts of the acid H₃PO₄ and a salt of its conjugate base NaH₂PO₄.

酸H₃PO₄と、その共役塩基の塩NaH₂PO₄を等量含む溶液に少量の酸や塩基を加えても、pHが大きく変わらない理由を説明しなさい。

Solution解き方

Excess H₃O⁺ is removed primarily by the reaction:

H_{3} O^{+} (a q) + H_{2} {PO}_{4}^{−} (a q) ⟶ H_{3} {PO}_{4} (a q) + H_{2} O (l)

Excess base is removed by the reaction:

{OH}^{−} (a q) + H_{3} {PO}_{4} (a q) ⟶ H_{2} {PO}_{4}^{−} (a q) + H_{2} O (l)

過剰なH₃O⁺は、主に次の反応で除かれる。

H_{3} O^{+} (a q) + H_{2} {PO}_{4}^{−} (a q) ⟶ H_{3} {PO}_{4} (a q) + H_{2} O (l)

　過剰な塩基は次の反応で除かれる。

{OH}^{−} (a q) + H_{3} {PO}_{4} (a q) ⟶ H_{2} {PO}_{4}^{−} (a q) + H_{2} O (l)

Exercise 79問題 79

Explain why the pH does not change significantly when a small amount of an acid or a base is added to a solution that contains equal amounts of the base NH₃ and a salt of its conjugate acid NH₄Cl.

塩基NH₃と、その共役酸の塩NH₄Clを等量含む溶液に少量の酸や塩基を加えても、pHが大きく変わらない理由を説明しなさい。

Exercise 80問題 80

What is [H₃O⁺] in a solution of 0.25 M CH₃CO₂H and 0.030 M NaCH₃CO₂?

{CH}_{3} {CO}_{2} H (a q) + H_{2} O (l) ⇌ H_{3} O^{+} (a q) + {CH}_{3} {CO}_{2}^{−} (a q) K_{a} = 1.8 \times 10^{−5}

CH₃CO₂Hが0.25 M、NaCH₃CO₂が0.030 Mの溶液の[H₃O⁺]はいくらか。

{CH}_{3} {CO}_{2} H (a q) + H_{2} O (l) ⇌ H_{3} O^{+} (a q) + {CH}_{3} {CO}_{2}^{−} (a q) K_{a} = 1.8 \times 10^{−5}

Solution解き方

[H₃O⁺] = 1.5

\times

10⁻⁴ M

[H₃O⁺] = 1.5

\times

10⁻⁴ M

Exercise 81問題 81

What is [H₃O⁺] in a solution of 0.075 M HNO₂ and 0.030 M NaNO₂?

{HNO}_{2} (a q) + H_{2} O (l) ⇌ H_{3} O^{+} (a q) + {NO}_{2}^{−} (a q) K_{a} = 4.5 \times 10^{−5}

HNO₂が0.075 M、NaNO₂が0.030 Mの溶液の[H₃O⁺]はいくらか。

{HNO}_{2} (a q) + H_{2} O (l) ⇌ H_{3} O^{+} (a q) + {NO}_{2}^{−} (a q) K_{a} = 4.5 \times 10^{−5}

Exercise 82問題 82

What is [OH⁻] in a solution of 0.125 M CH₃NH₂ and 0.130 M CH₃NH₃Cl?

{CH}_{3} {NH}_{2} (a q) + H_{2} O (l) ⇌ {CH}_{3} {NH}_{3}^{+} (a q) + {OH}^{−} (a q) K_{b} = 4.4 \times 10^{−4}

CH₃NH₂が0.125 M、CH₃NH₃Clが0.130 Mの溶液の[OH⁻]はいくらか。

{CH}_{3} {NH}_{2} (a q) + H_{2} O (l) ⇌ {CH}_{3} {NH}_{3}^{+} (a q) + {OH}^{−} (a q) K_{b} = 4.4 \times 10^{−4}

Solution解き方

[OH⁻] = 4.2

\times

10⁻⁴ M

[OH⁻] = 4.2

\times

10⁻⁴ M

Exercise 83問題 83

What is [OH⁻] in a solution of 1.25 M NH₃ and 0.78 M NH₄NO₃?

{NH}_{3} (a q) + H_{2} O (l) ⇌ {NH}_{4}^{+} (a q) + {OH}^{−} (a q) K_{b} = 1.8 \times 10^{−5}

NH₃が1.25 M、NH₄NO₃が0.78 Mの溶液の[OH⁻]はいくらか。

{NH}_{3} (a q) + H_{2} O (l) ⇌ {NH}_{4}^{+} (a q) + {OH}^{−} (a q) K_{b} = 1.8 \times 10^{−5}

Exercise 84問題 84

What is the effect on the concentration of acetic acid, hydronium ion, and acetate ion when the following are added to an acidic buffer solution of equal concentrations of acetic acid and sodium acetate:

等しい濃度の酢酸と酢酸ナトリウムからなる酸性緩衝液に、次の物質を加えると、酢酸、オキソニウムイオン、酢酸イオンの濃度にどのような影響があるか。

(a) HCl

(b) KCH₃CO₂

(d) KOH

(e) CH₃CO₂H

Solution解き方

(a) The added HCl will increase the concentration of H₃O⁺ slightly, which will react with

{CH}_{3} {CO}_{2}^{−}

and produce CH₃CO₂H in the process. Thus,

[{CH}_{3} {CO}_{2}^{−}]

decreases and [CH₃CO₂H] increases. (b) The added KCH₃CO₂ will increase the concentration of

[{CH}_{3} {CO}_{2}^{−}]

which will react with H₃O⁺ and produce CH₃CO₂ H in the process. Thus, [H₃O⁺] decreases slightly and [CH₃CO₂H] increases. (c) The added NaCl will have no effect on the concentration of the ions. (d) The added KOH will produce OH⁻ ions, which will react with the H₃O⁺, thus reducing [H₃O⁺]. Some additional CH₃CO₂H will dissociate, producing

[{CH}_{3} {CO}_{2}^{−}]

ions in the process. Thus, [CH₃CO₂H] decreases slightly and

[{CH}_{3} {CO}_{2}^{−}]

increases. (e) The added CH₃CO₂H will increase its concentration, causing more of it to dissociate and producing more

[{CH}_{3} {CO}_{2}^{−}]

and H₃O⁺ in the process. Thus, [H₃O⁺] increases slightly and

[{CH}_{3} {CO}_{2}^{−}]

increases.

(a) HClを加えるとH₃O⁺の濃度がわずかに増え、

{CH}_{3} {CO}_{2}^{−}

と反応してCH₃CO₂Hを生じる。したがって、

[{CH}_{3} {CO}_{2}^{−}]

は減り、[CH₃CO₂H]は増える。(b) KCH₃CO₂を加えると

[{CH}_{3} {CO}_{2}^{−}]

の濃度が増え、H₃O⁺と反応してCH₃CO₂Hを生じる。したがって、[H₃O⁺]はわずかに減り、[CH₃CO₂H]は増える。(c) NaClを加えてもイオン濃度には影響しない。(d) KOHを加えるとOH⁻イオンが生じ、H₃O⁺と反応して[H₃O⁺]を下げる。さらにCH₃CO₂Hの一部が解離して、

[{CH}_{3} {CO}_{2}^{−}]

イオンを生じる。したがって、[CH₃CO₂H]はわずかに減り、

[{CH}_{3} {CO}_{2}^{−}]

は増える。(e) CH₃CO₂Hを加えるとその濃度が増え、より多くが解離して、

[{CH}_{3} {CO}_{2}^{−}]

とH₃O⁺がさらに生じる。したがって、[H₃O⁺]はわずかに増え、

[{CH}_{3} {CO}_{2}^{−}]

も増える。

Exercise 85問題 85

What is the effect on the concentration of ammonia, hydroxide ion, and ammonium ion when the following are added to a basic buffer solution of equal concentrations of ammonia and ammonium nitrate:

等しい濃度のアンモニアと硝酸アンモニウムからなる塩基性緩衝液に、次の物質を加えると、アンモニア、水酸化物イオン、アンモニウムイオンの濃度にどのような影響があるか。

(a) KI

(b) NH₃

(d) NaOH

(e) NH₄Cl

Exercise 86問題 86

What will be the pH of a buffer solution prepared from 0.20 mol NH₃, 0.40 mol NH₄NO₃, and just enough water to give 1.00 L of solution?

0.20 molのNH₃、0.40 molのNH₄NO₃に、溶液全体が1.00 Lになるだけの水を加えて作った緩衝液のpHはいくらか。

Solution解き方

pH = 8.95

Exercise 87問題 87

Calculate the pH of a buffer solution prepared from 0.155 mol of phosphoric acid, 0.250 mole of KH₂PO₄, and enough water to make 0.500 L of solution.

0.155 molのリン酸、0.250 molのKH₂PO₄に、溶液全体が0.500 Lになるだけの水を加えて作った緩衝液のpHを計算しなさい。

Exercise 88問題 88

How much solid NaCH₃CO₂•3H₂O must be added to 0.300 L of a 0.50-M acetic acid solution to give a buffer with a pH of 5.00? (Hint: Assume a negligible change in volume as the solid is added.)

0.50 M酢酸水溶液0.300 LからpH 5.00の緩衝液を作るには、固体NaCH₃CO₂•3H₂Oをどれだけ加えればよいか。（ヒント：固体を加えることによる体積変化は無視できると仮定する。）

Solution解き方

37 g (0.27 mol)

Exercise 89問題 89

What mass of NH₄Cl must be added to 0.750 L of a 0.100-M solution of NH₃ to give a buffer solution with a pH of 9.26? (Hint: Assume a negligible change in volume as the solid is added.)

0.100 MのNH₃水溶液0.750 LからpH 9.26の緩衝液を作るには、NH₄Clを何g加えればよいか。（ヒント：固体を加えることによる体積変化は無視できると仮定する。）

Exercise 90問題 90

A buffer solution is prepared from equal volumes of 0.200 M acetic acid and 0.600 M sodium acetate. Use 1.80

\times

10⁻⁵ as K_a for acetic acid.

0.200 M酢酸水溶液と0.600 M酢酸ナトリウム水溶液を等体積混ぜて緩衝液を作る。酢酸のK_aには1.80

\times

10⁻⁵を使いなさい。

(a) What is the pH of the solution?

(a) 溶液のpHはいくらか。

(b) Is the solution acidic or basic?

(b) この溶液は酸性か塩基性か。

Solution解き方

(a) pH = 5.222; (b) The solution is acidic. (c) pH = 5.220

(a) pH = 5.222；(b) 酸性である；(c) pH = 5.220

Exercise 91問題 91

A 5.36–g sample of NH₄Cl was added to 25.0 mL of 1.00 M NaOH and the resulting solution

5.36 gのNH₄Clを、1.00 M NaOH水溶液25.0 mLに加え、得られた溶液を

diluted to 0.100 L.

0.100 Lになるまで希釈した。

(a) What is the pH of this buffer solution?

(a) この緩衝液のpHはいくらか。

(b) Is the solution acidic or basic?

(b) この溶液は酸性か塩基性か。

buffer capacity

緩衝容量

amount of an acid or base that can be added to a volume of a buffer solution before its pH changes significantly (usually by one pH unit)

一定体積の緩衝液のpHが大きく（通常は1）変わるまでに加えられる酸または塩基の量

buffer

緩衝液

mixture of appreciable amounts of a weak acid-base pair. The pH of a buffer resists change when small amounts of acid or base are added

弱い酸塩基対を十分な量だけ含む混合物。少量の酸や塩基を加えてもpHが変化しにくい

Henderson-Hasselbalch equation

ヘンダーソン–ハッセルバルヒの式

logarithmic version of the acid ionization constant expression, conveniently formatted for calculating the pH of buffer solutions

酸の電離定数の式を対数で表し、緩衝液のpHを計算しやすい形にしたもの