Titrations

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CognitoTitration — method and calculations

Key facts & methods

Titration method

1. Use a pipette to transfer a precise volume (e.g. 25.0 cm³) of the alkali into a conical flask.
2. Add a few drops of indicator (phenolphthalein or methyl orange).
3. Fill the burette with acid. Record the initial burette reading.
4. Add acid slowly from burette. Swirl the flask constantly.
5. Near the endpoint, add acid dropwise. Stop when the indicator permanently changes colour.
6. Record final burette reading. Titre = final − initial reading.
7. Repeat until two concordant results obtained (within 0.10 cm³ of each other). Use the mean.

Indicators and endpoints

Phenolphthalein: pink in alkali → colourless in acid. Use for strong acid/strong or weak alkali.
Methyl orange: yellow in alkali → red in acid. Use for strong acid/strong or weak base.
At the endpoint, one drop causes a permanent colour change.
Concordant results: two titres that agree to within 0.10 cm³ — these are reliable.
Rough titre: first, fast run-through. Used to identify approximate endpoint. NOT included in mean.

Titration calculation

1. Calculate mean titre (cm³). Convert to dm³ (÷1000).
2. Calculate moles of the known solution: n = c × V.
3. Use the equation ratio to find moles of the unknown.
4. Calculate concentration: c = n ÷ V.
Example: HCl + NaOH → NaCl + H₂O (1:1 ratio). 25.0 cm³ NaOH, concentration unknown. Mean titre of 0.1 mol/dm³ HCl = 22.5 cm³.
Moles HCl = 0.1 × 0.0225 = 0.00225 mol. Moles NaOH = 0.00225 mol. Concentration NaOH = 0.00225 ÷ 0.025 = 0.09 mol/dm³.

Exam questions — 5 questions · 12 marks · AQA 8462 style

4 marksTitration calculationAQA 8462 P2 style 🔢 Calculator

25.0 cm³ of sodium hydroxide solution of unknown concentration required 18.6 cm³ of 0.1 mol/dm³ hydrochloric acid to neutralise it. The equation is: NaOH + HCl → NaCl + H₂O. Calculate the concentration of the sodium hydroxide. (4 marks)

Hint: Step 1: moles HCl = c × V (convert cm³ to dm³). Step 2: use equation ratio (1:1) to get moles NaOH. Step 3: c(NaOH) = moles ÷ V(NaOH).

+40 XP

1 markIndicator choiceAQA 8462 style

Which indicator would you use for a titration of sodium hydroxide with hydrochloric acid, and what colour change indicates the endpoint?

A Litmus — blue to red

B Phenolphthalein — pink to colourless

C Phenolphthalein — colourless to pink

D Universal indicator — green to red

3 marksDescribe methodAQA 8462 P2 style

Describe how you would carry out a titration to find the volume of acid needed to neutralise 25.0 cm³ of alkali. Include how you would ensure your results are reliable. (4 marks)

Hint: Cover: pipette for alkali, burette for acid, indicator, swirling, dropwise near endpoint, repeated titrations, concordant results, mean.

+30 XP

Titration method

Indicators and endpoints

Titration calculation

Module complete! 🎉