When a solute is added to a solvent the freezing point of the solution is lower than that of the pure solvent. The lowering of freezing point is very small. A chemist called Beckmann invented a thermometer capable of measuring these small temperature changes accurately. The Beckmann thermometer must be calibrated at the start of the experiment. An incomplete diagram of the Beckmann apparatus is shown containing pure liquid cyclohexane, an organic solvent with a freezing point of about 6.5°C. The diagram does not show how the cyclohexane could be frozen. [Figure 1.1: Diagram with Beckmann thermometer, stirring wire, stopper, boiling tube, pure liquid cyclohexane] The method for determining the lowering of freezing point is as follows. step 1 Add 20.00 g of pure liquid cyclohexane to a clean dry boiling tube. step 2 Place the stopper containing the Beckmann thermometer and stirring wire into the boiling tube. step 3 Cool the pure cyclohexane. When it starts to freeze, set the Beckmann thermometer to 0.00 to calibrate it. step 4 Allow the pure cyclohexane to melt. Remove the stopper from the boiling tube. Add 0.250g of an organic solid X to the pure cyclohexane and replace the stopper. Stir the solution to dissolve X and refreeze the solution. Record the new freezing point. step 5 Allow the solution to melt. Remove the stopper from the boiling tube. Add a further known mass of X to the solution and replace the stopper. Stir the solution to dissolve X and refreeze the solution. Record the new freezing point. step 6 Repeat step 5 until sufficient readings are obtained.

1. (a)Complete the diagram to show how the pure liquid cyclohexane could be frozen using simple laboratory apparatus.[1]
2. (b)(i)In step 1, the cyclohexane can be measured using an electronic balance, a beaker and a clean dry boiling tube as shown. [Figure 1.2: Diagram showing electronic balance, beaker to support boiling tube, boiling tube] Describe a suitable method to add precisely 20.00 g of cyclohexane to the boiling tube. Assume that the balance is accurate to two decimal places and that common laboratory apparatus is available.[1]
3. (b)(ii)Alternatively in step 1, the volume of cyclohexane with a mass of exactly 20.00 g can be measured and added to the boiling tube. Calculate the volume of cyclohexane with a mass of precisely 20.00 g. The density of cyclohexane is 0.78 g cm⁻³. Give your answer to two decimal places. volume of cyclohexane ........................ cm³ Explain whether a burette is suitable for measuring this volume.[2]
4. (b)(iii)In step 4 the mass of X is measured on an electronic balance accurate to three decimal places before adding it to the cyclohexane. A student suggests the following technique. • An empty container is placed on the electronic balance. • The mass of the empty container is recorded to three decimal places. • 0.250 g of X is added to the container. • X is tipped from the container into the cyclohexane. Explain why this technique would not be accurate for adding 0.250 g of X to the cyclohexane.[1]
5. (c)(i)A ratio, B, is calculated as follows. B = mass of X (g) / mass of solvent (g) Complete the table by calculating B for each reading. Give your answers to three significant figures. | reading number | total mass of X added to 20.00 g of cyclohexane/g | B | ΔTfp/°C | |---|---|---|---| | 1 | 0.250 | | 1.35 | | 2 | 0.400 | | 2.20 | | 3 | 0.500 | | 2.75 | | 4 | 0.800 | | 4.40 | | 5 | 0.950 | | 5.30 | | 6 | 1.150 | | 6.40 | | 7 | 1.300 | | 7.25 | | 8 | 1.400 | | 8.50 |[2]
6. (c)(ii)Plot a graph on the grid to show the relationship between B and ΔTfp. Draw the line of best fit. [Figure 1.3: Graph grid with axes labeled ΔTfp/°C and B][2]
7. (c)(iii)Identify, by the reading number, the single most anomalous point. Suggest what error in the experiment could have caused this anomaly. reading number ........................ reason ..........................................................................................[1]
8. (c)(iv)In another experiment, a student added an unknown mass of X to 20.00 g of cyclohexane and measured ΔTfp as 5.00 °C. Use your graph to determine the mass of X used in this experiment. mass of X = ........................ g[2]
9. (c)(v)Determine the gradient of your line of best fit. State the coordinates of the two points you used for your calculation. coordinates 1 ................................ coordinates 2 ................................ gradient = ................................ °C[2]
10. (d)ΔTfp is related to the Mr of X by the following expression ΔTfp = KB / Mr where B = mass of X (g) / mass of solvent (g) K = a constant The Mr of X can be found using the gradient of your line of best fit. Mr = K / gradient The numerical value of K is 20020. Use this value for K and the gradient you determined in (c)(v) to calculate the Mr of X. Give your answer to the nearest whole number. If you were unable to calculate the gradient in (c)(v), assume that the gradient is 103°C. This is not the correct value. Mr = ........................[1]
11. (e)A student used the Beckmann apparatus and repeated the experiment described in (b) with an unknown solid Y. The student found the Mr of Y to be 136. Y is an aromatic carboxylic acid. Suggest the structure of Y. [Ar: C, 12.0; O, 16.0; H, 1.0][1]

This question uses a diagram from the original paper, which isn't shown here. Open the full paper from the past papers archive to see it.