Core pH Equations
\( \text{pH} = -\log_{10}[\text{H}^+] \qquad [\text{H}^+] = 10^{-\text{pH}} \)
\( \text{pOH} = -\log_{10}[\text{OH}^-] \qquad \text{pH} + \text{pOH} = 14 \text{ (at 25°C)} \)
\( K_w = [\text{H}^+][\text{OH}^-] = 1.00 \times 10^{-14} \text{ at 25°C} \)
Worked Example
Find the pH of 0.020 mol dm⁻³ NaOH
NaOH is a strong base: [OH⁻] = 0.020 mol dm⁻³
pOH = −log(0.020) = 1.70
pH = 14 − 1.70 = 12.30
Titration Curves
Strong Acid + Strong Base Titration
Buffer Solutions (HL)
A buffer resists changes in pH when small amounts of acid or base are added. Made from a weak acid + its conjugate base (or vice versa).
At the half-equivalence point: \( \text{pH} = pK_a \)
Think About It
Why is the equivalence point pH = 7 for strong acid/strong base but NOT 7 for weak acid/strong base?
At the equivalence point of a weak acid/strong base titration, the solution contains the conjugate base (e.g. CH₃COO⁻), which hydrolyses in water to produce OH⁻. This makes the solution basic (pH > 7).