Chemistry 9701/52 — May/June 2023

Cambridge A-Level · Planning, Analysis and Evaluation · worked solutions for every part, with the mark scheme

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Topics Analysis, Conclusions and Evaluation · Planning

Q1Analysis, Conclusions and EvaluationPlanningFree sample

Three students are asked to prepare samples of benzoic acid, $\text{C}_6\text{H}_5\text{COOH}$ , from the alkaline hydrolysis of methyl benzoate, $\text{C}_6\text{H}_5\text{COOCH}_3$ . Sodium benzoate, $\text{C}_6\text{H}_5\text{COONa}$ , is the salt formed, which is then acidified with hydrochloric acid to form $\text{C}_6\text{H}_5\text{COOH}$ .

\text{C}_6\text{H}_5\text{COOCH}_3(\text{l}) + \text{NaOH}(\text{aq}) \rightarrow \text{C}_6\text{H}_5\text{COONa}(\text{aq}) + \text{CH}_3\text{OH}(\text{l})

\text{C}_6\text{H}_5\text{COONa}(\text{aq}) + \text{HCl}(\text{aq}) \rightarrow \text{C}_6\text{H}_5\text{COOH}(\text{s}) + \text{NaCl}(\text{aq})

The students use the following method.

step 1 Put $1.242\text{ g}$ of liquid methyl benzoate into a $50\text{ cm}^3$ round-bottomed flask.

step 2 Prepare $100.0\text{ cm}^3$ of $1.00\text{ mol dm}^{-3}$ aqueous sodium hydroxide, $\text{NaOH}(\text{aq})$ .

step 3 Add $10\text{ cm}^3$ , an excess, of the prepared $\text{NaOH}(\text{aq})$ to the round-bottomed flask.

step 4 Add some anti-bumping granules to the round-bottomed flask.

step 5 Fit a condenser to make reflux apparatus. Reflux the reaction mixture for 20 minutes.

step 6 Allow the reaction mixture to cool and carefully pour the liquid into a beaker.

step 7 Acidify the liquid with dilute hydrochloric acid.

step 8 Filter the mixture formed in step 7.

step 9 Purify the benzoic acid by recrystallisation from hot water. Filter, dry and record the mass of pure benzoic acid obtained.

(a)

(i)

Calculate the volume of methyl benzoate used in step 1.
Give your answer to the nearest $0.05\text{ cm}^3$ .

[density: methyl benzoate, $1.08\text{ g cm}^{-3}$ ]

(ii)

Identify a suitable piece of apparatus to measure the volume of methyl benzoate required in step 1.

(iii)

Calculate the mass of $\text{NaOH}(\text{s})$ that is needed to prepare the solution in step 2.

(b)

Student 1 added the mass of $\text{NaOH}(\text{s})$ calculated in (a)(iii) into a beaker.

Describe the steps the student should take to make $100.0\text{ cm}^3$ of $1.00\text{ mol dm}^{-3}$ $\text{NaOH}(\text{aq})$ .

Give the name and capacity of any apparatus that should be used.
Write your answer using a series of numbered steps.

(c)

Student 2 prepared $0.100\text{ mol dm}^{-3}$ $\text{NaOH}(\text{aq})$ instead of $1.00\text{ mol dm}^{-3}$ $\text{NaOH}(\text{aq})$ in step 2.

State how this would affect the final mass of benzoic acid formed. Explain, using calculations, how you came to this conclusion.

[ $M_r$ : methyl benzoate, 136; benzoic acid, 122]

(d)

(i)

Explain why it is necessary to reflux the mixture in step 5.

(ii)

Explain why a naked flame is not used in step 5.

(iii)

Explain the purpose of transferring the liquid in step 6.

(e)

(i)

Explain what the students should do to confirm that the mixture has been acidified in step 7.

(ii)

Describe what you would expect to observe as the sodium benzoate mixture is acidified in step 7.

(f)

Suggest why it is necessary to cool the mixture before filtering in step 9.

(g)

Pure benzoic acid has a melting point of $122\text{ }^\circ\text{C}$ . The product made by student 1 has a melting point of $119\text{ }^\circ\text{C}$ .

This student suggests the melting point of the product was lower than expected because it contained some water.

Explain what the student should do to ensure that the product no longer contains water.

(h)

(i)

Calculate the maximum mass of benzoic acid that can be formed from $1.242\text{ g}$ of methyl benzoate.

[ $M_r$ : methyl benzoate, 136; benzoic acid, 122]

(ii)

Student 3 produces $0.825\text{ g}$ of benzoic acid from $1.242\text{ g}$ of methyl benzoate.

Calculate the percentage yield of benzoic acid produced by student 3.

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