Required practical
Determination of the reacting volumes of solutions of a strong acid and a strong alkali by titration
This required practical involves using appropriate apparatus to make and record a range of measurements accurately, including The volume of a three-dimensional shape is a measure of the amount of space or capacity it occupies, eg an average can of fizzy drink has a volume of 330 ml. of liquids. It also involves determining the The concentration of a solution tells us how much of a substance is dissolved in water. The higher the concentration, the more particles of the substance are present. of one of the Mixture formed by a solute and a solvent. in mol/dm3 and g/dm3.
It is important in this practical to:
- make and record accurate measurements
- take account of health and safety considerations
The description below describes one way to carry out the practical. Eye protection must be worn.
Aims
- To determine the reacting volumes of solutions of an acid and alkali by titration.
- To determine the concentration of one of the solutions in mol/dm3 and g/dm3 (Higher tier only).
Planning the practical
The plan needs to address these points:
- which apparatus to use
- which reagents to use
- a consideration of hazards, risks and precautions
Apparatus
The required apparatus is:
- A piece of apparatus used to add varying but measured volumes of solution during a titration.
- A piece of apparatus used to measure accurate and repeatable volumes of liquid. Also called a volumetric pipette. and filler
- conical flask and white tile
Reagents
The reagents are:
- an acid (hydrochloric, sulfuric or nitric) of unknown concentration
- sodium hydroxide solution of known concentration
- a suitable indicator, for example methyl orange or phenolphthalein
Hazards, risks and precautions
Identify the hazards and suggest precautions needed to reduce the risk of harm. For example:
| Hazard | Possible harm | Precaution |
| Dilute sodium hydroxide solution | Causes skin and serious eye irritation | Wear gloves and eye protection, and use a pipette filler |
| Spilling hydrochloric acid while filling the burette | Causes eye irritation | Fill the burette slowly below eye level, using a funnel |
| Hazard | Dilute sodium hydroxide solution |
|---|---|
| Possible harm | Causes skin and serious eye irritation |
| Precaution | Wear gloves and eye protection, and use a pipette filler |
| Hazard | Spilling hydrochloric acid while filling the burette |
|---|---|
| Possible harm | Causes eye irritation |
| Precaution | Fill the burette slowly below eye level, using a funnel |
This table does not include all the possible hazards.
Carrying out the practical
- Use the pipette and pipette filler to add 25 cm3 of alkali to a clean conical flask.
- Add a few drops of indicator and put the conical flask on a white tile.
- Fill the burette with acid and note the starting volume.
- Slowly add the acid from the burette to the alkali in the conical flask, swirling to mix.
- Stop adding the acid when the In a titration, the point at which the indicator first permanently changes colour. is reached (when the indicator first permanently changes colour). Note the final volume reading.
- Repeat steps 1 to 5 until you get Titres within 0.10 cm鲁 of each other. (see step 1 in the Analysis). More accurate results are obtained if acid is added drop by drop near to the end-point.
Results
Record the results in a suitable table, such as the one below. This table also shows some sample readings.
| Run | End vol | Start vol | Titre |
| Rough | 25.45 cm3 | 0.00 cm3 | 25.45 cm3 |
| 1 | 24.80 cm3 | 1.00 cm3 | 23.80 cm3 鉁 |
| 2 | 47.90 cm3 | 23.80 cm3 | 24.10 cm3 |
| 3 | 23.70 cm3 | 0.00 cm3 | 23.70 cm3 鉁 |
| Run | Rough |
|---|---|
| End vol | 25.45 cm3 |
| Start vol | 0.00 cm3 |
| Titre | 25.45 cm3 |
| Run | 1 |
|---|---|
| End vol | 24.80 cm3 |
| Start vol | 1.00 cm3 |
| Titre | 23.80 cm3 鉁 |
| Run | 2 |
|---|---|
| End vol | 47.90 cm3 |
| Start vol | 23.80 cm3 |
| Titre | 24.10 cm3 |
| Run | 3 |
|---|---|
| End vol | 23.70 cm3 |
| Start vol | 0.00 cm3 |
| Titre | 23.70 cm3 鉁 |
Readings should be recorded to two decimal places, ending in 0 or 5 (where the liquid level is between two graduations on the burette). The Volume of one reactant needed to react completely with the other reactant in a titration. is the volume added (the difference between the end and start readings).
Analysis
- At least two concordant titres should be ticked (鉁). These are titres within 0.20 cm3 (or sometimes 0.10 cm3) of each other.
- Calculate the mean titre. This is the volume of acid that exactly reacts with the sodium hydroxide solution of unknown concentration.
Evaluating the practical
The evaluation should address these points:
- What actions were taken to make accurate measurements?
- What improvements could be made to the method?
- Was the practical carried out safely? What improvements could be made?
Question
Describe four steps needed to obtain accurate results.
- Make sure the burette is vertical.
- Take the readings from the bottom of the meniscus.
- Near to the end-point, rinse the inside of the flask with distilled water.
- Add the acid drop by drop.
Question
Explain why a pipette is used to measure the alkali, rather than a measuring cylinder.
The pipette allows the same volume of alkali to be added each time, helping to make the results repeatable.