Section 1 of 7Aim and equipment
Aim
- Aim To find how pH changes when a fixed volume of dilute hydrochloric acid is neutralised by adding measured amounts of an insoluble base such as calcium oxide or calcium hydroxide.
Equipment
- Equipment Dilute hydrochloric acid (0.5 or 1 mol/dm3), solid base, conical flask, 25 cm3 pipette or measuring cylinder, spatula, weighing boat and glass rod.
- A pH probe, or universal indicator paper with a colour chart.
Section 2 of 7Method
1
Measure a fixed volume of the dilute acid into a conical flask using a pipette.
2
Record the starting pH using a probe, or by dipping a clean glass rod into the acid and touching it on universal indicator paper.
3
Add one spatula measure of the base and swirl until it has stopped reacting.
4
Record the pH again, using a clean rod for indicator paper each time.
5
Repeat, adding one spatula at a time, up to about ten, always using the same volume and concentration of acid.
6
Record all the results in a table.
Section 3 of 7Variables
| Type | Variable |
|---|---|
| Independent | Number of spatulas (amount) of solid base added |
| Dependent | pH of the solution |
| Control | Volume and concentration of acid, same base, same method of measuring pH, temperature |
Section 4 of 7Analysis
1Plot a graph
- Plot a graph Amount of base on the x-axis and pH on the y-axis.
2Initial rise
- Initial rise pH slowly rises as acid is used up by the base.
3Steep section
- Steep section pH jumps rapidly near neutral, where the last of the acid is neutralised.
4Plateau
- Plateau Beyond neutralisation the pH levels off at a high value because the excess base cannot dissolve any further.
- If 6 spatulas are needed to reach the steep rise, that is the amount required to neutralise the acid.
Section 5 of 7Hazards and precautions
Chemical hazards
- Chemical hazards Calcium oxide and calcium hydroxide are irritants to eyes and skin;
- avoid breathing the powder.
- Dilute hydrochloric acid at this concentration is low hazard but may irritate eyes and skin.
Precautions
- Precautions Wear eye protection, avoid skin contact, and wash off any spills with plenty of water.
Improving accuracy
- Improving accuracy A pH probe gives more reliable readings than indicator paper, which relies on judging a colour.
Section 6 of 7Key terms
| Term | Meaning |
|---|---|
| Neutralisation | A reaction between an acid and a base that forms a salt and water. |
| pH probe | An electronic sensor that measures pH on a meter. |
| Base | A substance that reacts with an acid to form a salt and water. |
| End point | The point at which the acid has just been neutralised. |
Section 7 of 7Exam technique
1
Reusing the same glass rod without rinsing, which contaminates later pH readings.
2
Changing the volume of acid between readings, so results cannot be compared.
3
Adding the next amount of base before the previous one has finished reacting.
4
Describing the steep part of the graph as an error rather than the neutralisation point.