# Acid-base titrations

> Chemistry · IB SL
> Source: https://www.owlsprep.com/study/ib-chemistry-sl-u6-acid-base-titrations/

We cover titration lab procedures, volumetric calculations, pH curve interpretation, indicator selection, and common mark traps for IB Chemistry SL.

**Prerequisites:** [Mole stoichiometry for aqueous solutions](https://www.owlsprep.com/study/ib-chemistry-sl-u1-molar-calculations/); [Properties of strong and weak acids and bases](https://www.owlsprep.com/study/ib-chemistry-sl-u6-acid-base-properties/)

## Learning objectives

- Describe standard acid-base titration procedures and correct use of volumetric glassware
- Perform stoichiometric calculations to find unknown concentrations of acids or bases
- Interpret key features of pH titration curves for all strong/weak acid-base combinations
- Select appropriate indicators for different titration types using pKa values

## Titration Concentration Calculations

All titration calculations follow a 4-step stoichiometric workflow, using the average of your concordant titre volumes to reduce random error. You must use the mole ratio from the balanced reaction equation to relate moles of acid and base.

**Titration calculation formula** — Where $n_a$ and $n_b$ are the stoichiometric coefficients of the acid and base in the balanced reaction equation.

*Notation:* $c_a V_a / c_b V_b = n_a / n_b$

*Example:* For the 1:1 reaction HCl + NaOH → NaCl + H₂O, $n_a = n_b$ so $c_a V_a = c_b V_b$.

**Worked example:** A 25.0 cm³ sample of 0.120 mol dm⁻³ H₂SO₄ is titrated against unknown KOH solution. The average concordant titre is 24.6 cm³. Find the concentration of KOH.

1. Write the balanced reaction equation to get the mole ratio:

   $$H_2SO_4 + 2KOH \rightarrow K_2SO_4 + 2H_2O$$
2. Calculate moles of H₂SO₄, converting volume to dm³:

   $$n(H_2SO_4) = 0.120 \times (25.0 / 1000) = 0.00300 \text{ mol}$$
3. Use 1:2 mole ratio to find moles of KOH:

   $$n(KOH) = 2 \times 0.00300 = 0.00600 \text{ mol}$$
4. Calculate concentration of KOH using titre volume:

   $$c(KOH) = 0.00600 / (24.6 / 1000) = 0.244 \text{ mol dm}^{-3}$$

*Calculator:* allowed

## Common pitfalls

- **Wrong:** Forgetting to convert cm³ to dm³ before calculating moles
  - Why it fails: Standard concentration units are mol dm⁻³, so volume must be divided by 1000 to match units
  - Correct: Always convert all volume values to dm³ before performing any titration calculation
- **Wrong:** Rinsing the conical flask with the analyte solution
  - Why it fails: Residual analyte adds unmeasured extra moles, leading to artificially high titre volumes
  - Correct: Rinse the conical flask only with distilled water; residual water does not change total moles of analyte
- **Wrong:** Assuming all equivalence points have a pH of 7
  - Why it fails: Only strong acid-strong base titrations produce a neutral salt at equivalence; other combinations make acidic or alkaline salts
  - Correct: Use the identity of the salt formed at the equivalence point to determine if pH is below, equal to, or above 7
- **Wrong:** Using phenolphthalein for a strong acid-weak base titration
  - Why it fails: Phenolphthalein changes colour above pH 8, outside the steep vertical jump of this titration curve
  - Correct: Use methyl orange, whose 3.1-4.4 pH range falls fully within the strong acid-weak base titration jump
- **Wrong:** Stopping the titration at the first faint colour change and not repeating
  - Why it fails: Single titre values often have large random error, leading to incorrect final concentration values
  - Correct: Repeat titrations until you get two concordant titres within 0.1 cm³, then use the average of these two values

## Cheatsheet

| Titration Type | Equivalence pH | Suitable Indicator | Indicator pKa |
| --- | --- | --- | --- |
| Strong acid + Strong base | 7 | Methyl orange / Phenolphthalein | 3.7 / 9.3 |
| Strong acid + Weak base | <7 | Methyl orange | 3.7 |
| Weak acid + Strong base | >7 | Phenolphthalein | 9.3 |
| Weak acid + Weak base | ~7 | No suitable indicator | N/A |

## What's next

Mastering acid-base titrations gives you a foundational quantitative skill that is tested across 3 separate IB Chemistry SL assessment objectives, from practical Paper 2 skills to Paper 2 extended response calculations. This knowledge directly transfers to related acid-base equilibrium topics, including buffer solution behaviour and pH calculation for salt solutions. You will also use the same volumetric analysis workflow later in the course for redox titrations, which are a common 6+ mark extended question on past papers. Solidify this knowledge by working through 2-3 past paper titration calculation questions to lock in your workflow before moving on to more advanced equilibrium content.

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