A student carried out a series of titrations to determine the concentration of a standard sodium carbonate solution. The student titrated 20.0 cm3 samples of the sodium carbonate solution against dilute hydrochloric acid.
The results of the four titrations are shown in the table below.
| Titration 1 | Titration 2 | Titration 3 | Titration 4 | |
|---|---|---|---|---|
| Final burette reading (cm3^{3}3) | 21 | 21.55 | 20.95 | 23.15 |
| Initial burette reading (cm3^{3}3) | 0 | 1.20 | 0.50 | 1.85 |
| Volume of acid added (cm3^{3}3) | 21 | 20.35 | 20.45 | 21.30 |
Two of the titrations should not be used to calculate the mean volume of acid required.
Identify these two titrations and give a reason why each should be excluded from the calculation.
157 exam-style questions on Edexcel GCSE Chemistry 6.2 Quantitative analysis, covering 6.2.1 Concentration in mol dm⁻³ and titration calculations, 6.2.2 Percentage yield and why actual yield falls short, 6.2.3 Atom economy of a reaction, 6.2.4 Choosing a reaction pathway, and 6.2.5 Molar volume of gases and Avogadro's law. Each one has a worked solution and a mark scheme showing where the marks go.