🔬 Apparatus
Burette and stand, pipette and pipette filler, conical flask, white tile, small funnel, beakers, indicator, hydrochloric acid and sodium hydroxide solution, distilled water.
Reactions of Acids » Required Practical: Titration
What you'll learn this session
Study time: 30 minutes
AQA spec: 8.2.2 (Required practical 2)
- The method for finding the reacting volumes of a strong acid and a strong alkali by titration
- How to record and check your results
- How to avoid the mistakes examiners see most
- Higher tier: finding a concentration in mol/dm3 and g/dm3 from your results
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Unlock This CourseWhat this practical is about
In this required practical you find the volumes of a strong acid and a strong alkali that react together exactly. You will use hydrochloric acid (a strong acid) and sodium hydroxide solution (a strong alkali). The idea of a titration is explained in the lesson called Titrations. Here we focus on doing it well, step by step, and getting results you can trust.
This practical is a chance to show you can use apparatus to make and record accurate measurements, including the volume of liquids. It is also how you determine the concentration of a strong acid or a strong alkali.
Key terms:
- Rough titration: a quick first run used to find roughly where the end point is.
- Meniscus: the curved surface of a liquid in a narrow tube. You read the bottom of the curve.
Apparatus and safety
⚠ Safety
Wear eye protection. Sodium hydroxide is corrosive and can harm eyes and skin. Wipe up spills at once and rinse splashes with plenty of water. Never use your mouth to fill a pipette. Take the funnel out of the burette before you start.
The method
The white tile under the flask makes that colour change at the end point easy to spot
- Rinse the burette with a little of the acid. Rinse the pipette with a little of the alkali. This stops water from diluting your solutions.
- Use the funnel to fill the burette with acid. Open the tap briefly so the tip below the tap fills with acid and has no air bubble. Remove the funnel.
- Record the starting reading to the nearest 0.05 cm3. Read at eye level, at the bottom of the meniscus.
- Use the pipette and filler to measure 25.0 cm3 of the alkali into the conical flask.
- Add a few drops of indicator. Put the flask on the white tile so the colour change is easy to see.
- Do a rough titration. Add the acid fairly quickly, swirling all the time, until the colour changes. Record the final reading.
- Repeat with a fresh 25.0 cm3 of alkali. This time add the acid quickly until about 2 cm3 before the rough end point. Then add it drop by drop, swirling, until one drop changes the colour.
- Record the final reading. Subtract the start reading to get the volume of acid used.
- Repeat until you have two or more results that agree closely.
Recording your results
Draw a results table before you start. Give each column a heading with units, for example "Final reading (cm3)", "Start reading (cm3)" and "Volume of acid used (cm3)". Record every burette reading to two decimal places, ending in 0 or 5.
Find the mean using only the concordant results. Leave out the rough titration and any result that does not agree. Concordant results are explained in the lesson called Titrations.
Worked example
Volumes of acid used for 25.0 cm3 of alkali: rough 23.10, run 2 22.45, run 3 22.40, run 4 22.35 (all cm3).
Leave out the rough value. Runs 2, 3 and 4 are within 0.10 cm3 of each other, so use all three.
Mean = (22.45 + 22.40 + 22.35) ÷ 3 = 67.20 ÷ 3 = 22.40 cm3.
So 22.40 cm3 of acid reacts exactly with 25.0 cm3 of alkali.
Why each step matters
💧 Rinsing
Rinsing with the solution you will put in stops water changing its concentration. Never rinse the conical flask with the alkali, because extra alkali would change the amount in it.
🌀 Swirling
Swirling mixes the solutions, so the colour change shows up where the reaction is happening.
⏩ Drop by drop
One extra drop is a tiny volume. Adding drops near the end stops you overshooting the end point.
A conical flask can be rinsed with distilled water, because water does not change the number of moles of alkali you put in. The pipette and burette should not be left wet with water, because that would dilute the solution they hold. Rinse them with the solution they will hold.
Higher tier: finding a concentration
Higher tier only
You can use the mean titre and the known concentration of one solution to find the concentration of the other, in mol/dm3 and g/dm3.
Example: 25.0 cm3 of sodium hydroxide solution of unknown concentration needs a mean of 22.40 cm3 of 0.150 mol/dm3 hydrochloric acid.
Step 1: moles of HCl = 0.150 × 22.40 ÷ 1000 = 0.00336 mol.
Step 2: the equation NaOH + HCl → NaCl + H2O shows 1 mol reacts with 1 mol, so moles of NaOH = 0.00336 mol.
Step 3: concentration of NaOH = 0.00336 ÷ (25.0 ÷ 1000) = 0.134 mol/dm3 (3 significant figures).
Step 4: the relative formula mass of NaOH is 40, so the concentration is 0.1344 × 40 = 5.4 g/dm3 (2 significant figures).
Sources of error
Read the burette at eye level, at the bottom of the meniscus, or your titre will be off
- Overshooting the end point, so the titre is too big. Add drops slowly near the end.
- Reading the burette from the wrong angle, or at the top of the meniscus.
- Not filling the burette tip, so an air bubble is pushed out and the reading is wrong.
- Leaving the funnel in the burette, which can drip into it.
- Using too much indicator, which can change the result because it is itself a weak acid or alkali.
Common mistakes
Including the rough titration in the mean. Giving the final burette reading instead of the volume used (final minus start). Rinsing the pipette with water instead of the alkali. Forgetting units in the table headings.
Exam-style question
A student does a titration. They use a pipette to put 25.0 cm3 of sodium hydroxide solution into a conical flask and add acid from a burette. Their titres are 21.60, 20.85, 20.80 and 20.90 cm3. (a) Which result should the student not use to work out the mean? Explain why. (b) Calculate the mean titre. (c) Give one way the student can make sure they add the acid accurately near the end point.
Model answer
(a) 21.60 cm3, because it is not within 0.10 cm3 of the other results, so it is not concordant (it was probably the rough titration).
(b) (20.85 + 20.80 + 20.90) ÷ 3 = 62.55 ÷ 3 = 20.85 cm3.
(c) Add the acid drop by drop while swirling the flask, until one drop changes the colour.
Exam tip
If a question asks why you do a rough titration, say it shows roughly where the end point is, so the later runs can be done carefully and accurately.