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

A student is given two separate unlabelled aqueous solutions, A and B.
The student knows the solutions are magnesium chloride and barium chloride.

The student carries out a series of test-tube reactions to identify the two solutions. The results are recorded in Table 1.

Table 1

Use the results in Table 1 to identify the cation in Solution A and the cation in Solution B.

Give an ionic equation, including state symbols, for the reaction that occurs in Test 2 with Solution B.

The student wishes to confirm the identity of the anion in the solutions.

Describe a chemical test the student could carry out to confirm the presence of chloride ions in one of the solutions.

State the observation the student would make.

Barium chloride solution is used as a reagent to test for sulfate ions.
The student is told that the barium chloride reagent must be acidified before use.

Suggest a suitable acid and explain why it must be added.

A student prepared a sample of ethanal (CH₃CHO) by the oxidation of ethanol. The student purified the ethanal using the distillation apparatus shown in Figure 1.

Figure 1

The apparatus in Figure 1 is set up incorrectly.

Identify three mistakes in the student’s setup. For each mistake, explain why it is a problem or a safety risk.

Ethanal has a boiling point of 21 ^oC. The reaction also produces ethanoic acid, which has a boiling point of 118 ^oC.

Explain why distillation is a suitable technique to separate ethanal from ethanoic acid.

After distillation, the impure ethanal is shaken with a solution of sodium carbonate in a separating funnel. Two layers form.

State the purpose of adding sodium carbonate solution.

The student collects 4.18 g of pure ethanal. The student started with 9.20 g of ethanol.

The overall equation for the reaction is:

CH₃CH₂OH + [O] → CH₃CHO + H₂O

Calculate the percentage yield of ethanal.

A second student suggests that a better method to check the purity of the collected liquid is to use gas chromatography.

Outline the stationary phase and mobile phase in gas chromatography and explain why a more volatile compound would have a shorter retention time.

The student wants to confirm the identity of the product using high-resolution mass spectrometry.

State the information that can be determined from the molecular ion peak in a high-resolution mass spectrum.

A student investigated the effect of temperature on the rate of the reaction between sodium thiosulfate and hydrochloric acid.

Na₂S₂O₃ (aq) + 2HCl (aq) → 2NaCl (aq) + SO₂ (g) + S (s)

The student carried out the reaction at five different temperatures. In each experiment, they measured the time, t, for a fixed amount of sulfur to be produced. The rate of reaction is considered to be proportional to 1/t.

The student's results are shown in Table 1.

Table 1

Complete Table 1 by calculating the values for Rate ∝ 1/t. Give your answers to three significant figures.

Plot a graph of Rate ∝ 1/t (y-axis) against Temperature (x-axis) on the grid provided. Draw a smooth curve of best fit.

Use your graph to determine the time, t, for the reaction to complete at 35 ^oC. Show your working on the graph.

Explain why a small increase in temperature has a large effect on the rate of this reaction.

A student carried out a titration to determine the concentration of a sample of hydrochloric acid. The student titrated 25.0 cm³ portions of the hydrochloric acid against a standard solution of 0.108 mol dm^-3 sodium hydroxide.

The titration results are shown in Table 1.

Table 1

The student used phenolphthalein as the indicator.

State the colour change that would be observed in the conical flask at the end point.

Before starting the titration, the student failed to fill the space below the burette tap (the jet).

State what effect this error would have on the value recorded for Titration 1. Explain your reasoning.

The student identified Titration 4 as being anomalous.

Suggest why the student identified this result as anomalous and should ignore it when calculating the mean titre.

Use the appropriate results from Table 1 to calculate the mean titre.

The burette used by the student has an uncertainty of ±0.05 cm³ for each reading.

Calculate the percentage uncertainty for the titre in Titration 2.

Use your answer from part d to calculate the concentration of the hydrochloric acid.

A student carried out a titration by adding 0.110 mol dm^-3 sodium hydroxide solution from a burette to 25.0 cm³ of a solution of propanoic acid (CH₃CH₂COOH) in a beaker. The pH of the mixture was measured throughout the titration.

The resulting pH curve is shown in Figure 1.

Figure 1

Write an equation for the reaction between propanoic acid and sodium hydroxide.

Use Figure 1 to determine the volume of sodium hydroxide solution added at the equivalence point.

Use this value to calculate the concentration of the propanoic acid solution.

Explain, with reference to the species present, why the solution acts as a buffer in the region between 4.0 cm³ and 16.0 cm³ of added sodium hydroxide.

The half-equivalence point is where half of the propanoic acid has been neutralised.

Use Figure 1 to determine the pK_a of propanoic acid and use this value to calculate the acid dissociation constant, K_a.

Explain why methyl orange (pH range 3.1 – 4.4) is not a suitable indicator for this titration.

A student investigated the electrode potentials of different metals by creating a series of electrochemical cells as shown in the simplified setup in Figure 1.

Figure 1

The student’s method involved using copper as Metal 1 and connecting it to the positive terminal of the voltmeter. Different metals (M) were used for Metal 2. The filter paper was soaked in potassium nitrate solution. The results are shown in Table 1.

Table 1

State why the metal strips are cleaned with sandpaper before setting up the cell.

The student observed that the reading for the silver cell was negative.

Explain what the negative EMF reading indicates about the polarity of the silver half-cell relative to the copper half-cell.

Explain why potassium nitrate is a suitable chemical for the soaked filter paper but potassium chloride would not be suitable for a cell containing a silver electrode.

The student states that this experimental method is not suitable for determining a precise standard electrode potential.

Give two reasons why the values obtained from this simplified experiment would be different from the standard electrode potentials found in data books.

A standard electrochemical cell made by combining a Cu²⁺ / Cu half-cell and an Mg²⁺ / Mg half-cell has a standard EMF of +2.71 V.

The standard electrode potential for the Cu²⁺ (aq) / Cu (s) half-cell is +0.34 V.

Give the conventional representation for this cell and use it to calculate the standard electrode potential for the Mg²⁺ (aq) / Mg (s) half-cell.

The student then changed the concentration of the magnesium sulfate solution from 1.0 mol dm^-3 to 0.1 mol dm^-3.

Predict how this change will affect the EMF of the cell. Explain your reasoning.

A student is given two aqueous solutions, X and Y. The student is told the solutions contain one of the following complex ions:

[Fe(H₂O)₆]²⁺

[Cu(H₂O)₆]²⁺

[Fe(H₂O)₆]³⁺

The student carries out a series of test-tube reactions on solutions X and Y. The observations are recorded in Table 1.

Table 1

Use the results in Table 1 to identify the complex ion present in Solution X and in Solution Y.

Give the ionic equation for the reaction that occurs in Test 3 with Solution Y.

Explain why effervescence is observed in Test 3 with Solution Y.

Give the formula of the complex ion responsible for the deep blue solution formed in Test 2 with Solution X.

A student carried out a calorimetry experiment to determine the enthalpy change of solution, ΔH_sol, for potassium chloride (KCl). They added 5.00 g of KCl to 100.0 cm³ of water and recorded a temperature decrease of 3.1 K.

The following thermodynamic data are available.

The M_r of KCl is 74.6.

The specific heat capacity of the solution is 4.18 J g^-1 K^-1.

Assume the density of the solution is 1.00 g cm⁻³.

Use the student's experimental results to calculate the enthalpy change of solution, ΔH_sol, for potassium chloride. Give your answer in kJ mol^-1, to an appropriate number of significant figures.

Calculate the standard entropy change of solution, ΔS^⦵_sol, for potassium chloride.

Use your answer from 01.1 and 01.2 to calculate the Gibbs free-energy change, ΔG^⦵, for the dissolving of potassium chloride at 298 K.

Use this value to explain why potassium chloride dissolves in water at this temperature.

A student used thin-layer chromatography (TLC) to identify the amino acids present in an unknown mixture, U. The stationary phase used was a polar silica plate and the mobile phase was a non-polar solvent.

The student ran chromatograms of three pure amino acids, Alanine (Ala), Leucine (Leu), Phenylalanine (Phe), and the unknown mixture U.

The resulting chromatogram is shown in Figure 1.

Figure 1

To ensure the results are accurate and reproducible, the student must follow a precise experimental procedure.

State two practical steps the student should take when setting up and running the chromatogram, and give a reason for each step.

Use Figure 1 to identify the amino acids present in mixture U.

Use Figure 1 to calculate the R_f value for Leucine (Leu).

Alanine is a more polar molecule than phenylalanine.

Explain, in terms of intermolecular forces and solubility, why phenylalanine travels further up the chromatogram than alanine.

Chlorambucil is a drug used in the treatment of leukaemia and other cancers is shown. It is a slow-acting drug and has the chemical structure:

Determine the molecular formula for chlorambucil.

Identify two functional groups found in this compound.

Explain what is the benefit of this drug being slow-acting?

Chlorambucil reacts with

• dilute sodium hydroxide

• ethanol with concentrated sulfuric acid

• potassium cyanide in the aqueous ethanol

For each of the reactions, draw the expected organic products of the reaction.

State the type of reaction, and outline the mechanism for the reaction between chlorambucil and potassium cyanide in ethanol.

At 25^oC chlorambucil has a pK_a of 4.9.

Determine the K_a for chlorambucil. Give your answer to three significant figures.

The pK_a of benzoic acid is 4.2 at the same temperature. Suggest a reason for the difference in acid strength between the two compounds.

Describe and explain how the K_a of chlorambucil could be determined starting from a 0.1 moldm^-3 solution.

You may use a diagram to support your answer.