Quick answer: Acids are substances that release hydrogen ions, H⁺, in water. This Chemistry 9-10 lab on acids and bases gives the aim, the equipment, the safety points, a step-by-step method, sample results and 8 analysis questions; the answers are on a separate page and the lab sheet is a free printable PDF.
Two acids can have the same concentration and still behave very differently. This lab compares hydrochloric acid with ethanoic acid by measuring pH and the time magnesium takes to react, then asks you to explain the results using ionisation.
Level: Chemistry grades 9-10 (GCSE · IGCSE · O Level · Class 9-10) · Curriculum: GCSE C4, IGCSE 7
Aim
To compare a strong acid with a weak acid of the same concentration by measuring pH and the rate of reaction with magnesium.
Equipment
- 0.100 mol/dm³ hydrochloric acid and 0.100 mol/dm³ ethanoic acid
- Two 100 cm³ beakers, two 10.0 cm³ measuring cylinders
- pH meter or universal indicator solution with a colour chart
- Magnesium ribbon, two pieces of 2.0 cm
- Stopwatch, safety goggles
Safety
Wear goggles. The acids are dilute, but wash splashes with water. Hydrogen gas is flammable, so keep naked flames away. Rinse the probe between solutions.
Method
- Measure 10.0 cm³ of hydrochloric acid into one beaker and 10.0 cm³ of ethanoic acid into the other.
- Measure the pH of each acid and record it.
- Drop a 2.0 cm piece of magnesium ribbon into the first beaker and start the stopwatch.
- Stop the watch when the ribbon has disappeared and record the time.
- Repeat with the second acid, using the same size of ribbon.
- Record the brightness of bubbling as fast, slow or very slow.
Results
| Measurement | Hydrochloric acid | Ethanoic acid |
|---|---|---|
| pH of 0.100 mol/dm³ | 1.0 | 2.9 |
| Time for 2.0 cm Mg to disappear (s) | 85 | 640 |
| Bubbling | Vigorous | Slow |
| Volume of 0.100 mol/dm³ NaOH to neutralise 10.0 cm³ (cm³) | 10.0 | 10.0 |
Analysis questions
Question 1 Compare the pH
State which acid has the lower pH and which has more H⁺ ions.Question 2 Hydrogen ion concentration
pH 1.0 means [H⁺] = 0.10 mol/dm³ and pH 2.9 means [H⁺] = 0.0013 mol/dm³. Calculate the percentage of ethanoic acid molecules that have ionised.Question 3 Rate of reaction
Explain why magnesium reacts faster with hydrochloric acid.Question 4 Equal neutralisation
Both acids need 10.0 cm³ of sodium hydroxide. Explain what this shows about the amount of acid in each beaker.Question 5 Strong or concentrated
State why the experiment keeps the concentration the same.Question 6 Control variables
Name three variables that must stay the same.Question 7 Sources of error
Suggest two sources of error in the timing and how to reduce them.Question 8 Extend the investigation
Describe how you would show that the weak acid still gives the same total amount of hydrogen.
