A student separates 50 cm3^{3}3 of a mixture of propanone and water by fractional distillation.
Propanone boils at 56 °C. Water boils at 100 °C.
The student records the temperature at the top of the fractionating column and the total volume of liquid collected.
The table shows the results.
| Time (min) | Temperature at top of column (°C) | Total volume of liquid collected (cm3^{3}3) |
|---|---|---|
| 0 | 20 | 0 |
| 2 | 41 | 0 |
| 4 | 56 | 3 |
| 6 | 56 | 9 |
| 8 | 56 | 15 |
| 10 | 57 | 19 |
| 12 | 79 | 20 |
| 14 | 100 | 23 |
| 16 | 100 | 29 |
| 18 | 100 | 35 |
Identify the liquid collected between 4 minutes and 8 minutes. Give a reason for your answer.
The student collects the liquid in three flasks.
Flask 1: 4 to 10 minutes
Flask 2: 10 to 14 minutes
Flask 3: 14 to 18 minutes
Explain why the liquid in Flask 2 is not pure.
Estimate the percentage by volume of propanone in the original mixture.
Explain why your answer is only an estimate.
Explain how the fractionating column stops water reaching the condenser between 4 minutes and 8 minutes.
Another student does the same experiment but puts the thermometer bulb in the liquid in the flask.
Explain why this student could not use the thermometer readings to decide when to change flasks.
51 exam-style questions on OCR GCSE Chemistry 2.1 Purity and separating mixtures, covering 2.1.1 Pure substances and melting point data, 2.1.2 Relative formula mass and empirical formula, 2.1.3 Formulations as useful mixtures, 2.1.4 Techniques for separating mixtures, and 2.1.5 Chromatography and Rf values. Each one has a worked solution and a mark scheme showing where the marks go.