Edexcel IGCSE Chemistry · Spec 2.33C
Acid-Alkali Titrations
How to carry out an acid-alkali titration accurately.
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Acid-Alkali Titrations
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Explained
Finding a concentration by titration
A titration finds an unknown concentration precisely, by measuring exactly how much of one solution reacts with a known volume of the other.
The apparatus, and why each piece
A pipette measures a fixed volume of the alkali into the conical flask. It is used rather than a measuring cylinder because it delivers one exact volume very accurately.
A burette holds the acid. It is graduated with a tap, so you can add acid slowly and read off precisely how much has gone in.
A conical flask rather than a beaker, because it can be swirled without splashing. A white tile underneath, because the colour change is far easier to see against white.
An indicator, a few drops only. Too much would itself react with the acid and shift the result.
The method
- Rinse the burette with the acid and the pipette with the alkali, so no water dilutes them.
- Pipette a measured volume of alkali into the flask and add a few drops of indicator.
- Fill the burette, run the tap briefly to fill the tip below the tap, and record the starting reading.
- Add acid while swirling, quickly at first, then drop by drop near the end point.
- Stop at the first permanent colour change and record the final reading. The titre is the difference between the two readings.
- Repeat until you have concordant results, meaning ones within 0.10 cm cubed of each other, then take the mean of those.
What the mark scheme accepts and rejects
Edexcel International GCSE Chemistry mark schemes are unusually exact about this practical, because almost every mark is a precision mark.
Asked to name the apparatus that measures the fixed volume, one credits a pipette. A later paper is stricter and wants a volumetric pipette, rejecting a dropping pipette. The word volumetric is doing work: a dropping pipette measures nothing.
Asked why a white tile is used, it credits that the colour change can be seen more clearly.
On reading the burette, it states that each value must be given to two decimal places. A burette is read to the nearest 0.05 cm cubed, so 27.4 is an incomplete reading and 27.40 is a complete one. The trailing zero is not decoration; it records the precision of the instrument.
On the mean, the notes say an error carried forward is allowed if different results have been ticked, but that there is no error carried forward if no results are ticked at all. Marking which titres are concordant is therefore part of the answer, not working you can leave out.
One further note, from a tick box question on apparatus: extra ticks in any row lose a mark. Ticking widely to be safe costs marks rather than gaining them.
Using the result
Work in moles. Find the moles of the solution you know, using concentration multiplied by volume in cubic decimetres, remembering that 1000 cm cubed is 1 dm cubed.
Use the balanced equation to convert to moles of the unknown. If one mole reacts with one mole the number is unchanged, but a reaction such as sulfuric acid with sodium hydroxide is one to two, and that ratio is where marks are lost.
Then divide the moles of the unknown by its volume in cubic decimetres to give its concentration.
Why the first titre is discarded
The first run is a rough one, done quickly to find roughly where the end point falls. It is usually overshot, so it is not included in the mean. Later runs, approached drop by drop near that known point, are the accurate ones.
Spec 2.33C
What you need to know
- Describe the titration apparatus
- Carry out a titration step by step
- Use the result to find a concentration
Active recall
Quick check
Answer each question before opening the answer.
What is a titration used to find out?
The exact volume of acid and alkali that react together, used to calculate an unknown concentration.
Why is an indicator used in a titration?
It changes colour at the end-point to show when neutralisation is complete.
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