Titration Calculations
A concise revision guide to concentrations in mol dm⁻³ and g dm⁻³, carrying out an acid–base titration with methyl orange or phenolphthalein, concordant titres, and the calculation steps from titre to unknown concentration.
- 5.11-a
- 5.11-b
- 5.11-c
- 5.11-d
- 5.11-e
What these spec points say
- 5.11-a be able to calculate solution concentrations in mol dm-3
- 5.11-b be able to calculate solution concentrations in g dm-3
- 5.11-c be able to carry out simple acid-base titration calculations using a range of acids, alkalis and indicators
- 5.11-d know the use of phenolphthalein as an indicator
- 5.11-e know the use of methyl orange as an indicator
Titration to the End Point
Choose a step to watch an acid–alkali titration, from filling the burette to a set of concordant titres.
© Dr. Mohammed Al-Fatah – onlinelearningsystem.net
GCSE Recap: Moles in Solution
Three quick questions on concentration, volume and moles before the titration calculations begin.
Concentration in mol dm⁻³ and g dm⁻³
The concentration of a solution is the amount of solute in one cubic decimetre of solution. In mol dm⁻³ it is the moles per dm³; in g dm⁻³ it is the mass per dm³. The two are linked by the molar mass:
concentration in g dm⁻³ = concentration in mol dm⁻³ × Mr
Because volumes in the laboratory are measured in cm³, the volume must be divided by 1000 before it is used: n = c × V ÷ 1000, with V in cm³.
Worked example: 4.00 g of sodium hydroxide (Mr 40.0) is dissolved to make 250 cm³ of solution.
n = 4.00 ÷ 40.0 = 0.100 mol; c = 0.100 ÷ 0.250 = 0.400 mol dm⁻³, which is 0.400 × 40.0 = 16.0 g dm⁻³.
Unit check: cm³ ÷ 1000 = dm³. A titre of 23.45 cm³ is 0.02345 dm³.
Quick Check: Convert Between the Units
Five rapid questions: pick the right value for each solution as quickly as you can.
Carrying Out an Acid–Base Titration
A titration finds the volume of one solution that exactly reacts with a measured volume of another.
A pipette delivers a fixed volume of one solution (usually 25.0 cm³) into a conical flask. The other solution is run in from a burette until the indicator changes colour at the end point.
The first titration is rough. Accurate titrations are then repeated until two concordant titres, within 0.10 cm³ of each other, are obtained and averaged.
The burette, pipette and conical flask, and a results table showing which titres are concordant and which are discarded.
| Indicator | Colour in acid | Colour in alkali | End point |
|---|---|---|---|
| Methyl orange | red | yellow | orange |
| Phenolphthalein | colourless | pink | first permanent pale pink (or the pink just disappears) |
Either indicator suits a strong acid with a strong base. Only a few drops are used, because indicators are themselves weak acids and too much would react with the alkali.
Technique point: Rinse the burette with the solution it will hold and the pipette with the solution it will deliver, but rinse the conical flask with distilled water only: any solution left in the flask would add extra moles.
Quick Check: Titration Practice
Put the steps of a titration in order and decide which titres from a results table are concordant.
Titration Calculations
The calculation always follows the same four steps. Write the equation, find the moles of the solution whose concentration is known, use the equation ratio to get the moles of the other solution, then divide by its volume in dm³.
Worked example: 25.0 cm³ of sodium hydroxide solution needed 22.30 cm³ of 0.100 mol dm⁻³ hydrochloric acid for neutralisation with methyl orange. Find the concentration of the sodium hydroxide.
- NaOH + HCl → NaCl + H₂O, so the ratio is 1 : 1.
- n(HCl) = 0.100 × 22.30 ÷ 1000 = 2.23 × 10⁻³ mol.
- n(NaOH) = 2.23 × 10⁻³ mol.
- c(NaOH) = 2.23 × 10⁻³ ÷ 0.0250 = 0.0892 mol dm⁻³.
With a diprotic acid the ratio changes: 2NaOH + H₂SO₄ → Na₂SO₄ + 2H₂O, so the moles of sodium hydroxide are twice the moles of sulfuric acid.
Common mistake: Using the pipette volume with the burette concentration. Match each volume to its own solution before multiplying.
Every titration calculation follows the same four steps: equation and ratio, moles of the known solution, moles of the unknown, then concentration.
Quick Check: A Titration Calculation
Calculate a concentration from titration results for an acid and base pair not used in the worked example, then convert it to g dm⁻³.
Quick Check: The Whole Page in Four Rounds
Pick the one accurate statement in each round to pull the page together.
FAQs
Use these quick answers to check the titration calculations.
How do I convert mol dm⁻³ to g dm⁻³?
Multiply the concentration in mol dm⁻³ by the molar mass of the solute.
What are concordant titres?
Accurate titres within 0.10 cm³ of each other. Only these are averaged; the rough titre is never included.
Which indicator should I use?
Methyl orange or phenolphthalein for strong acid against strong base; phenolphthalein for a weak acid against a strong base; methyl orange for a strong acid against a weak base.
Why is the conical flask rinsed with distilled water only?
The pipette fixes the moles placed in the flask. Water adds no moles, but leftover solution would add extra moles and make the titre too large.
How do I handle a 2 : 1 ratio such as sodium hydroxide with sulfuric acid?
Use 2NaOH + H₂SO₄ → Na₂SO₄ + 2H₂O. The moles of sodium hydroxide are twice the moles of sulfuric acid, so apply the factor of 2 before dividing by the volume.
Copyright notice: This OLS revision content, including the explanations, layout, diagrams, tables and embedded learning structure, is authored for Online Learning System by Dr. Mohammed Al-Fatah. It may not be copied, reproduced, redistributed or adapted without written permission.
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