Titration: Finding an Unknown Concentration
Run an acid and alkali titration to a single indicator end point, then use the mole ratio in the equation to calculate the unknown concentration from concordant titres.
What a learner can do afterwards
- Describes the apparatus and the steps, including rinsing, swirling, and repeating until titres agree
- Selects the concordant titres and takes the mean, discarding the rough run
- Calculates moles from concentration and volume and crosses the mole ratio to the other reactant
- Names one source of error and says which way it pushes the calculated concentration
1 · Read
A titration finds an unknown concentration from one you know. A burette holds acid of known concentration. A pipette measures a fixed volume of alkali, often 25.0 cm3, into a flask with a few drops of indicator. Rinse the burette and the pipette with the solutions they will hold, and rinse the flask with water. Then drip acid in with swirling until the colour flips. Run one rough titration first to find the endpoint zone.
Repeat until the titres agree within 0.1 cm3. Those agreeing titres are concordant. The rough run never joins the mean. With a rough 26.0 plus 25.20, 25.25 and 25.15, drop the rough and average the rest. Add 25.20, 25.25 and 25.15 to get 75.60, then divide by 3 to get 25.20 cm3.
Turn the mean titre into an answer. Moles equal concentration times volume in dm3. Suppose 25.0 cm3 of alkali needed 20.0 cm3 of 0.15 mol/dm3 acid in a 1 to 1 reaction. Acid moles are 0.15 times 0.0200, which is 0.003. The ratio gives 0.003 mol of alkali. Divide by 0.025 dm3 to get 0.12 mol/dm3. The maths trusts the titre completely: overshoot the endpoint and the counted moles swell, so the calculated concentration comes out too high.
Name each error and the way it pushes the result. Overshooting the endpoint inflates the titre, so the calculated concentration comes out too high. A burette rinsed with water dilutes the acid, so a bigger titre is needed and the result comes out too high as well. Technique decides accuracy, so go drop by drop near the end.
Rinse, drip to the endpoint, average the concordant titres, and turn the mean into moles and back.
The rewritten readings were written for this dataset.
More to read
Measuring a titre carefully
Rinse and fill the burette with acid; rinse the pipette with alkali. Pipette alkali into a flask and add indicator. Swirl the flask over a white tile while dripping acid until the indicator changes colour. Flask water is fine. Run a rough trial; repeat slowly near the endpoint for titres within 0.1 cm3.
Calculating the unknown concentration
Average concordant titres without the rough trial. Calculate moles as concentration × volume, with volume in dm3. Use the balanced equation's ratio to find moles of the unknown substance. Divide by its solution volume for concentration. Keep track of dm3, mol and mol/dm3.
Explaining titration errors
When known acid measures unknown alkali, overshooting raises the titre and calculated concentration. Rinse water left inside the burette dilutes acid, causing larger titres and an overestimate too. Contamination can obscure the endpoint. Report each error's direction, higher or lower, together with the measurement's limits.
2 · Watch
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Where it sits
Where this leads
Jobs that lean on this skill. Follow one to see everything it is built on.
11 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.