A student investigates four acid solutions, P, Q, R and S.
The student measures the pH of each solution with a pH meter.
The student then adds a 0.01 g piece of magnesium ribbon to 50 cm³ of each solution and measures the time for the magnesium to react completely.
The table shows the results.
| Solution | Acid | Concentration of acid (mol/dm³) | pH | Time for magnesium to react (s) |
|---|---|---|---|---|
| P | hydrochloric acid | 0.10 | 1 | 25 |
| Q | ethanoic acid | 0.10 | 3 | 280 |
| R | hydrochloric acid | 0.0010 | 3 | did not react completely |
| S | ethanoic acid | 1.0 | 2 | 70 |
Calculate how many times greater the concentration of hydrogen ions is in solution P than in solution Q.
Give two reasons why a pH meter is better than universal indicator for this investigation.
The student says:
"Solutions Q and R have the same pH, so they contain the same amount of acid and will react with magnesium in the same way."
Evaluate the student's statement.
Use the data and your knowledge of strong, weak, concentrated and dilute acids.
112 exam-style questions on OCR GCSE Chemistry 3.3 Types of chemical reactions, covering 3.3.1 Oxidation and reduction in terms of oxygen, 3.3.2 Acids, alkalis and neutralisation, 3.3.3 The pH scale and measuring pH, 3.3.4 Oxidation and reduction in terms of electrons, 3.3.5 Dilute and concentrated, weak and strong acids, and 3.3.6 Hydrogen ion concentration and pH. Each one has a worked solution and a mark scheme showing where the marks go.