Acids, Bases, pH and Buffers
Acids, Bases, pH and Buffers
The exam tests Brønsted-Lowry definitions, strong versus weak acid calculations, Kw, titration curves and indicator choice, buffer calculations and salt hydrolysis.
Core relationships
An acid is a proton donor, a base a proton acceptor; conjugate pairs differ by H+. pH = -log[H+], and [H+] = 10-pH. Kw = [H+][OH-] = 1.0 x 10-14 at 298 K, so in pure water pH = 7. For a strong monoprotic acid, [H+] equals the acid concentration; for a strong base, [OH-] equals the base concentration and [H+] = Kw/[OH-]. For a weak acid, Ka = [H+][A-]/[HA] and pKa = -log Ka; since dissociation is slight, [H+] is approximately the square root of (Ka x c).
Buffers and titration curves
A buffer resists pH change on adding small amounts of acid or base and consists of a weak acid with its conjugate base (or a weak base with its conjugate acid). pH = pKa + log([A-]/[HA]). Adding equal amounts of acid and salt gives pH = pKa. Titration curves show the vertical region and equivalence pH: strong acid-strong base equivalence at 7, weak acid-strong base above 7, weak base-strong acid below 7. Choose an indicator whose end-point range falls within the vertical section: phenolphthalein for weak acid-strong base, methyl orange for weak base-strong acid.
Worked example
Find the pH of 0.100 mol dm-3 ethanoic acid, Ka = 1.8 x 10-5. [H+] = square root of (1.8 x 10-5 x 0.100) = square root of (1.8 x 10-6) = 1.34 x 10-3 mol dm-3, so pH = -log(1.34 x 10-3) = 2.87. Do not simply take pH = 1.00, which would be the strong-acid answer. Mixing equal volumes of 0.100 mol dm-3 ethanoic acid with 0.100 mol dm-3 sodium ethanoate gives pH = pKa = 4.74, because dilution halves both components and their ratio is unchanged.
Traps
Treating a weak acid as fully dissociated; forgetting the square root for weak acids; using [H+] = Kw/[OH-] backwards for bases; saying a buffer must have equal concentrations; choosing an indicator by colour preference rather than end-point range; and ignoring salt hydrolysis, where the salt of a weak acid and strong base gives pH above 7.
Key ideas
- pH = -log[H+]; Kw = 1.0 x 10-14 at 298 K.
- Weak acid: [H+] = square root of (Ka x c).
- Buffer: pH = pKa + log([A-]/[HA]).
- Indicator range must lie within the titration curve's vertical section.
- Salt of weak acid with strong base is alkaline by hydrolysis.