A solution at 298 K has pH 4.0. What are [H+] and [OH−]?
- [H+] = 10−pH = 10−4.0 = 1.0 × 10−4 mol dm−3.
- [OH−] = Kw ÷ [H+] = (1.0 × 10−14) ÷ (1.0 × 10−4).
- [OH−] = 1.0 × 10−10 mol dm−3.
Answer: [H+] = 1.0 × 10−4 mol dm−3 and [OH−] = 1.0 × 10−10 mol dm−3
Proton transfer reactions
Chemistry · Reactivity 3: What are the mechanisms of chemical change?. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.
The idea
A Brønsted–Lowry acid donates a proton (H+) and a base accepts one. When an acid loses its proton, what is left is its conjugate base. The two differ by one H+. A species that can both donate and accept a proton, such as H2O or HCO3−, is amphiprotic.
pH measures [H+] on a log scale, so one pH unit is a tenfold change in [H+]. In water at 298 K, [H+] × [OH−] is always 1.00 × 10−14.
Strong acids and bases dissociate fully in water. Weak ones dissociate only partly, so at the same concentration they have fewer ions, a pH nearer 7, lower conductivity and slower reactions.
Rules to remember
- pH = −log10[H+] and [H+] = 10−pH, with [H+] in mol dm−3
- Kw = [H+][OH−] = 1.00 × 10−14 at 298 K
- Conjugate pair: acid and base differ by one H+, e.g. NH4+ / NH3 and H2O / OH−
- Strong acids: HCl, HNO3, H2SO4. Weak acids: ethanoic acid, carbonic acid
- Strong bases: NaOH, KOH. Weak base: NH3
- acid + base → salt + water; acid + carbonate or hydrogencarbonate → salt + water + CO2
- Strong acid with strong base: the pH curve has a steep vertical section with the equivalence point at pH 7
Common mistake
Students think a strong acid means a concentrated acid. Strong and weak describe how far the acid dissociates; concentrated and dilute describe how much acid is dissolved. Also, a change of 2 pH units is a factor of 100, not 2 or 20.
Worked example
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