Practical Techniques & Data Analysis (AQA AS Chemistry): Exam Questions

A student is given an unknown organic liquid, A, which is known to be one of the four compounds shown below.

butan-1-ol

butanal

butanone

butanoic acid

The student carries out a series of tests on sample A. The results are shown in the flow chart in Figure 1.

Figure 1

Use the results in Figure 1 to identify compound A.

The reagent used in Test 1 is acidified potassium dichromate(VI).

Give the formula of the active chromium species in the reagent before and after a positive test.

Describe the preparation of the Tollens' reagent used in Test 2.

A student carried out an experiment to determine the enthalpy of combustion of ethanol (C₂H₅OH). They burned a known mass of ethanol in a spirit burner and used the heat released to raise the temperature of a known volume of water in a copper calorimeter.

The student's results and relevant data are shown below.

Data

• M_r of ethanol = 46.0

• Specific heat capacity of water, c = 4.18 J g^-1 K^-1

• Density of water = 1.00 g cm^-3

• Accepted standard enthalpy of combustion of ethanol, ΔH_c^⦵ = –1367 kJ mol^-1

Use the student's results to calculate an experimental value for the enthalpy of combustion of ethanol, in kJ mol⁻¹.

Use your answer from part (a) and the accepted data book value to calculate the percentage error in the student's experimental result.

The student's calculated value is significantly less exothermic than the accepted data book value.

Suggest two major sources of error in this experimental method that could account for this difference.

A student carried out a series of experiments to determine the relative molecular mass, M_r, of a volatile organic liquid, Y.

In each experiment, a known mass of liquid Y was injected into a gas syringe which was heated to a constant temperature of 90 ^oC in an oven. The liquid vaporised, and the volume of the gas produced was recorded. The pressure in the laboratory was 101 kPa. The student's results are shown in Table 1.

Table 1

Use the results from the first experiment only (mass = 0.18 g) and the ideal gas equation, pV = nRT, to calculate a value for the M_r of liquid Y.

The gas constant R = 8.31 J K^-1 mol^-1.

Show your working.

On the grid provided, plot a graph of volume of gas against mass of liquid. Draw a line of best fit.

Use your line of best fit from part (b) to determine a more accurate value for the M_r of liquid Y. Show your working.

Explain why the value for the M_r calculated from the line of best fit is likely to be more accurate than the value calculated from the single experiment in part a.

Suggest two potential sources of error in this experimental method, other than measurement errors.

A student carried out an experiment to determine the enthalpy of solution (ΔH_sol) for sodium chloride (NaCl).

The student dissolved 2.34 g of NaCl in 50.0 cm³ of deionised water in a polystyrene cup and recorded a temperature change of –0.7 ^oC.

State one key procedural step, other than insulating the cup, that the student should have taken to ensure the measured temperature change was as accurate as possible.

Use the student's results to calculate the enthalpy of solution, ΔH_sol, for sodium chloride.

Give your answer in kJ mol^-1.

(M_r of NaCl = 58.5; specific heat capacity of water = 4.18 J g^-1 K^-1; assume the density of water is 1.00 g cm^-3)

The accepted data book value for the enthalpy of solution of NaCl is +3.9 kJ mol^-1.

Suggest one reason for the difference between the student's experimental value and the accepted value.