UnofficialAP ChemistryUnit 8 of 9

AP Chemistry Unit 8: Acids and Bases

pH, weak acids and bases, buffers and titrations.

Unit overview

Acid-base chemistry is quantitative. $\text{pH} = -\log[H_3O^+]$ and, at 25 °C, $\text{pH} + \text{pOH} = 14$ . Strong acids ionise completely, so their pH comes directly from concentration; weak acids reach an equilibrium described by $K_a$ .

Buffers contain a weak acid and its conjugate base and resist pH changes; the Henderson-Hasselbalch equation, $\text{pH} = \text{p}K_a + \log\frac{[A^-]}{[HA]}$ , gives their pH. Titration curves show the equivalence point and the half-equivalence point, where $\text{pH} = \text{p}K_a$ . Particulate diagrams are a common way to test whether you understand which species are present.

Key topics

pH and pOH: Logarithmic scale; each pH unit is a factor of 10 in $[H_3O^+]$ .
Weak acids and bases: $K_a$ , $K_b$ and $K_aK_b = K_w$ for conjugate pairs.
Buffers: Composition, capacity and the Henderson-Hasselbalch equation.
Titration curves: Equivalence and half-equivalence points and indicator choice.

Worked example

Find the pH of 0.001 M HCl.

HCl is a strong acid, so $[H_3O^+] = 0.001 = 10^{-3}$ M.
$\text{pH} = -\log(10^{-3}) = 3$ .

Answer: $\text{pH} = 3$

Common mistakes

Treating a weak acid as fully ionised.
Assuming the equivalence point is always at pH 7; it is above 7 for a weak acid titrated with a strong base.
Forgetting that diluting a buffer changes its capacity but barely changes its pH.

Exam tips

Write which species dominate at each stage of a titration before calculating.
Half-equivalence is the quickest way to read $\text{p}K_a$ from a curve.

Chemistry Core Formulas (6 flashcards)

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