The method and the results are on the acids and bases lab. Do the analysis yourself first, then check the answers below.
Quick answer: Acids are substances that release hydrogen ions, H⁺, in water. These answers go with the Chemistry 9-10 lab on acids and bases: the graph, the calculations, the uncertainties and the conclusion explained, for all 8 analysis questions.
The answers use the sample data. Your own pH and time values will differ, but the order of the results and the explanation in terms of ionisation and H⁺ concentration should match.
Level: Chemistry grades 9-10 (GCSE · IGCSE · O Level · Class 9-10) · Curriculum: GCSE C4, IGCSE 7
Hydrochloric acid (pH 1.0) has the lower pH, so it has more H⁺ ions than ethanoic acid (pH 2.9).
Question 2 Hydrogen ion concentration
0.0013 ÷ 0.100 × 100 = 1.3%. Hydrochloric acid, in contrast, is almost 100% ionised.
Question 3 Rate of reaction
The rate depends on the concentration of H⁺ ions. Hydrochloric acid has about 80 times more H⁺ ions (0.10 ÷ 0.0013), so collisions with magnesium are more frequent.
Question 4 Equal neutralisation
They contain the same number of moles of acid (0.00100 mol each). Ethanoic acid keeps ionising as H⁺ is used up, so it neutralises as much alkali as the strong acid.
Question 5 Strong or concentrated
So that any difference is due to the strength (degree of ionisation) of the acid and not to the amount dissolved.
Question 6 Control variables
Volume of acid, temperature and size of the magnesium ribbon (also its surface area and purity).
Question 7 Sources of error
Judging when the ribbon has gone is subjective, and the ribbon may differ in thickness. Repeat the test and use a mean, and cut the ribbon from the same strip.
Question 8 Extend the investigation
Collect the hydrogen in a gas syringe from excess magnesium with equal volumes of each acid. The final volumes will be the same because the moles of acid are the same.
