A student investigates how the pH of 50 cm3\mathrm{cm^{3}}cm3 of dilute hydrochloric acid changes when powdered calcium hydroxide is added.
The student adds 0.20 g of calcium hydroxide at a time. After each addition the student stirs the mixture and measures the pH with a pH meter.
The table shows the results.
| Total mass of calcium hydroxide added / g | pH |
|---|---|
| 0.00 | 1.2 |
| 0.20 | 1.3 |
| 0.40 | 1.5 |
| 0.60 | 1.8 |
| 0.80 | 2.5 |
| 1.00 | 11.6 |
| 1.20 | 12.0 |
| 1.40 | 12.1 |
Give the independent variable in this investigation.
Give two variables the student should control.
Describe how the pH changes as calcium hydroxide is added.
Estimate the mass of calcium hydroxide needed to neutralise the acid.
Write the ionic equation for the reaction between hydrogen ions and hydroxide ions.
Suggest one change to the method that would let the student find the mass needed for neutralisation more precisely.
44 exam-style questions on Edexcel GCSE Chemistry 4.1 Acids, covering 4.1.1 Acids, alkalis and the pH scale, 4.1.2 Hydrogen ion concentration and pH, 4.1.3 Dilute and concentrated, weak and strong acids, 4.1.4 Bases, alkalis and the reactions of acids, 4.1.5 Chemical tests for hydrogen and carbon dioxide, 4.1.6 Neutralisation as a reaction of acids with bases, 4.1.7 Preparing soluble salts, and 4.1.8 Solubility rules and preparing insoluble salts. Each one has a worked solution and a mark scheme showing where the marks go.