Acids, Bases, and Salts (including pH and buffers)
Acid-base questions combine equilibrium arithmetic with titration reasoning. You need pH and pOH for strong and weak acids and bases, salt hydrolysis, buffer design and the shape of titration curves, including indicator choice.
Core relationships
- K_w = [H+][OH-] = 1.0 × 10-14 at 25 °C, so pH + pOH = 14.
- For a weak acid, K_a = [H+]2/(c - [H+]); when dissociation is small, [H+] ≈ sqrt(K_a·c).
- Percentage dissociation = [H+]/c × 100, and it rises on dilution.
- K_a × K_b = K_w for a conjugate pair, so a weaker acid has a stronger conjugate base.
- Salt hydrolysis: a salt of a weak acid with a strong base gives pH > 7; a salt of a weak base with a strong acid gives pH < 7; strong/strong salts give pH 7.
- Buffers follow pH = pK_a + log([A-]/[HA]), work best near pH = pK_a, and are effective over roughly one unit either side.
Titration landmarks
Strong acid with strong base: equivalence at pH 7 with a steep jump. Weak acid with strong base: pH = pK_a at half-equivalence, and the equivalence point is basic. Weak base with strong acid mirrors this. Polyprotic acids such as H3PO4 show one buffer region and one steep step per acidic proton.
Worked example
A buffer holds 0.10 mol dm-3 ethanoic acid (pK_a 4.76) and 0.15 mol dm-3 sodium ethanoate. pH = 4.76 + log(0.15/0.10) = 4.76 + 0.18 = 4.94. Adding 0.010 mol of HCl to 1.00 dm3 converts 0.010 mol of ethanoate into acid, giving 0.11 and 0.14 mol dm-3: pH = 4.76 + log(0.14/0.11) = 4.86, a change of only 0.08 units.
Traps
- Using pH = -log c for a weak acid instead of the square-root approximation.
- Mixing K_a with pK_a, or reversing the buffer ratio.
- Assuming every titration has equivalence pH 7.
- Using initial moles in a buffer calculation without allowing for the new total volume when a strong acid is added.
- Skipping the approximation check: if more than about 5% dissociates, solve the quadratic.