Conjugate pairs and pH
Introduced| English | Meaning |
|---|---|
| conjugate base/ˈkɒndʒuːɡeɪt beɪs/ | The species formed when an acid loses one proton. |
| conjugate acid/ˈkɒndʒuːɡeɪt ˈæsɪd/ | The species formed when a base accepts a proton. |
| weak acid/wiːk ˈæsɪd/ | An acid that only partially ionises in aqueous solution. |
| strong acid/strɒŋ ˈæsɪd/ | An acid that essentially completely ionises in aqueous solution at the concentrations being considered. |
| strong alkali/strɒŋ ˈælkəlaɪ/ | A soluble base that dissociates almost completely, such as NaOH. |
Acids, bases and their partners
- When an acid loses $\text{H}^+$, what's left is its conjugate base 共轭碱.
- pH and the equilibrium constants put numbers on acidity.
- The method to find pH depends on the acid or base.
The conjugate base of CH₃COOH is:
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Removing one H⁺ from the acid leaves its conjugate base; the two differ by a single proton.
An acid and the base formed when it loses a proton are a conjugate ______.
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e.g. HA and A⁻.
Conjugate acid 共轭酸–base pairs
- An acid loses $\text{H}^+$ → its conjugate base; a base gains $\text{H}^+$ → its conjugate acid.
- The two differ by one $\text{H}^+$ — a conjugate pair. E.g. $\text{CH}_3\text{COOH}$ / $\text{CH}_3\text{COO}^-$.
Conjugate acid-base pairs differ by one proton
pH and H⁺ concentration
pH = −log[H⁺]: slide it and watch [H⁺] change tenfold per unit. A conjugate acid–base pair differ by a single proton.
pH is defined as:
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pH = −log[H⁺]; so [H⁺] = 10^(−pH).
A larger Ka (smaller pKa) means the acid is:
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A bigger acid dissociation constant means more dissociation, so a stronger acid.
Match each acid-base term.
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Each item links the term to its correct meaning.
pH and the constants
- $\text{pH} = -\log[\text{H}^+]$, so $[\text{H}^+] = 10^{-\text{pH}}$.
- weak acid 弱酸: $K_a = \dfrac{[\text{H}^+][\text{A}^-]}{[\text{HA}]}$, and $\text{p}K_a = -\log K_a$ (larger $K_a$ = stronger acid).
- water: $K_w = [\text{H}^+][\text{OH}^-] = 1.0 \times 10^{-14}$ at 298 K.
The pH scale: pH =-log of the hydrogen-ion concentration
To find [H⁺] for a weak acid, you use:
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A weak acid is only partly ionised, so [H⁺] = √(Ka × [HA]); a strong acid uses [H⁺] = concentration.
Calculating pH
- strong acid 强酸 (fully ionised): $[\text{H}^+]$ = acid concentration, then $-\log$.
- strong alkali 强碱: find $[\text{OH}^-]$, then $[\text{H}^+] = K_w / [\text{OH}^-]$.
- weak acid: $[\text{H}^+] = \sqrt{K_a \times [\text{HA}]}$.
Worked examples
1. $0.010$ \dfrac{\text{mol}}{\text{dm}^3} HCl is fully dissociated, so $[\text{H}^+] = 0.010$ and $\text{pH} = -\log_{10} 0.010 = 2.00$.
2. $0.10$ \dfrac{\text{mol}}{\text{dm}^3} ethanoic acid has $K_a = 1.7 \times 10^{-5}$. Assuming little dissociation, $[\text{H}^+] = \sqrt{K_a \times c} = \sqrt{1.7 \times 10^{-6}} = 1.3 \times 10^{-3}$, so $\text{pH} = 2.88$.
You've got it
- conjugate pair = acid and base differing by one $\text{H}^+$
- $\text{pH} = -\log[\text{H}^+]$; $K_a$ (bigger = stronger acid); $K_w = 1.0 \times 10^{-14}$
- strong acid: $[\text{H}^+]$ = conc; strong alkali: via $K_w$; weak acid: $\sqrt{K_a[\text{HA}]}$