Practical Techniques & Data Analysis (AQA AS 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 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.