Chemistry 20 · Acids and bases
pH is a logarithm
The whole pH scale exists because hydronium concentrations span fourteen orders of magnitude, and nobody wants to write 0.0000001 mol/L. Once you see it as a logarithm, the rest of the unit is definitions and one titration calculation.
- 1. The words, first
- 2. Two definitions, and why the second is better
- 3. The pH scale
- 4. Neutralization and titration
- 5. What costs marks
The words, first
The idea: Strong and concentrated are not the same thing, and mixing them up is the single most common error in this unit.
| Word | What it means |
|---|---|
| Arrhenius acid | Produces H⁺ in water, which immediately becomes H₃O⁺. |
| Arrhenius base | Produces OH⁻ in water. |
| Brønsted–Lowry acid | A proton donor. More general, because it needs no water. |
| Brønsted–Lowry base | A proton acceptor. |
| Conjugate base | What is left after an acid donates one proton. H₂SO₄ → HSO₄⁻. |
| Strong acid | Ionizes essentially completely. HCl, HBr, HI, HNO₃, H₂SO₄, HClO₄. |
| Weak acid | Ionizes only partly, so most of it stays as whole molecules. Acetic acid is the standard example. |
| Concentrated | A lot of solute per litre. Independent of strength — you can have dilute strong acid and concentrated weak acid. |
| pH | −log[H₃O⁺]. Each whole unit is a factor of ten. |
| pOH | −log[OH⁻]. At 25 °C, pH + pOH = 14. |
| Neutralization | Acid plus base giving a salt and water. |
| Titration | Adding a solution of known concentration until a reaction is exactly complete, to find an unknown concentration. |
| Equivalence point | Where the acid and base have reacted in exactly the stoichiometric ratio. The indicator's colour change marks it as the end point. |
Two definitions, and why the second is better
The idea: Arrhenius describes what happens in water. Brønsted–Lowry describes the proton transfer itself, which covers far more reactions.
Arrhenius. An acid releases H⁺, a base releases OH⁻. Perfectly serviceable for HCl and NaOH in water, and useless for ammonia, which is a base and contains no hydroxide.
Brønsted–Lowry. An acid donates a proton and a base accepts one. NH₃ + H₂O → NH₄⁺ + OH⁻ now makes sense: ammonia accepted a proton from water, so ammonia is the base and water is the acid.
Conjugate pairs. Every Brønsted acid leaves a conjugate base behind, differing by exactly one proton. In that reaction, NH₄⁺ is the conjugate acid of NH₃, and OH⁻ is the conjugate base of H₂O.
Water is amphiprotic — it can donate or accept, depending on what it is with. That is why it appears on both lists.
The pH scale
The idea: pH = −log[H₃O⁺]. Every whole unit down is ten times more hydronium, which is why a pH of 3 is a hundred times more acidic than a pH of 5.
Reading it directly. Whenever the concentration is a clean power of ten, the exponent is the pH: [H₃O⁺] = 1.0 × 10⁻⁵ → pH 5.
Strong acid. HCl ionizes completely, so 0.001 mol/L HCl has [H₃O⁺] = 1 × 10⁻³ and pH 3.
Strong base — two steps. For 0.010 mol/L NaOH, [OH⁻] = 0.010, so pOH = 2, and then pH = 14 − 2 = 12. Taking the logarithm and stopping gives 2, which is the pOH. This is the most common single error on pH questions.
Strength versus concentration. A 0.1 mol/L solution of a strong acid has a lower pH than a 0.1 mol/L solution of a weak one, because more of it has ionized. A concentrated weak acid can still be less acidic than a dilute strong one.
Neutralization and titration
The idea: A titration is a stoichiometry problem with a colour change to tell you when to stop. Find the amount you added, use the mole ratio, divide by the unknown volume.
Worked example. 25.0 mL of HCl is exactly neutralized by 20.0 mL of 0.100 mol/L NaOH.
- Amount of base: n = cV = (0.100)(0.0200) = 2.00 × 10⁻³ mol
- Mole ratio: HCl + NaOH → NaCl + H₂O, so 1:1, and the acid sample held the same amount.
- Concentration of acid: c = n/V = 2.00 × 10⁻³/0.0250 = 0.0800 mol/L
The ratio is not always 1:1. H₂SO₄ + 2NaOH → Na₂SO₄ + 2H₂O needs twice as much base per mole of acid. Balancing the equation before step two is not optional.
Why several trials. The first titration is usually rough. Concordant trials — within about 0.1 mL of each other — are averaged, and the rough one is discarded.
What costs marks
The idea: Two of these are conceptual and two are procedural.
- Stopping at pOH. If the question gives a base and asks for pH, there is one more step.
- Treating strong as a synonym for concentrated. They are independent.
- Assuming a 1:1 mole ratio in a titration. Balance first.
- Forgetting that pH is logarithmic. pH 4 is ten times more acidic than pH 5, not 25% more.