Chemistry 9701/52 — October/November 2022

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

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Topics Planning · Analysis, Conclusions and Evaluation

Q1PlanningAnalysis, Conclusions and EvaluationFree sample

Eggshells contain a high percentage by mass of calcium carbonate, $\text{CaCO}_3$ . A student wants to find out what percentage of an eggshell is calcium carbonate and uses the following method.

This method uses a known excess of acid to dissolve the eggshell. The amount of unreacted acid is then determined by titration with an alkali. Assume the acid only reacts with the $\text{CaCO}_3$ in the eggshell.

step 1 Wash an empty eggshell with distilled water.

step 2 Warm the eggshell in an oven for a few minutes until dry.

step 3 Grind the eggshell into a powder.

step 4 Weigh approximately $2\text{ g}$ of the eggshell powder into a conical flask using a balance which measures to three decimal places.

step 5 Add $100\text{ cm}^3$ of $2.00\text{ mol dm}^{-3}$ hydrochloric acid to the conical flask.

step 6 Loosely cover the conical flask and leave for two days.

step 7 Filter the contents of the conical flask, with any rinsings, into a $250.0\text{ cm}^3$ volumetric flask and top-up to the mark using distilled water.

step 8 Transfer $25.00\text{ cm}^3$ of the solution prepared in step 7 into a conical flask, add a few drops of thymol blue indicator and titrate against $1.00\text{ mol dm}^{-3}$ sodium hydroxide using a $50\text{ cm}^3$ burette.

The calcium carbonate in the eggshell reacts with the excess hydrochloric acid as follows.

\text{CaCO}_3(\text{s}) + 2\text{HCl}(\text{aq}) \rightarrow \text{CaCl}_2(\text{aq}) + \text{CO}_2(\text{g}) + \text{H}_2\text{O}(\text{l})

The excess acid reacts with the sodium hydroxide solution as follows.

\text{NaOH}(\text{aq}) + \text{HCl}(\text{aq}) \rightarrow \text{NaCl}(\text{aq}) + \text{H}_2\text{O}(\text{l})

(a)

(i)

Suggest how the student could confirm the eggshell is completely dry in step 2.

(ii)

State why the eggshell is made into a powder in step 3 before making up the solution. Explain your answer.

(iii)

Suggest why the solution is left for two days in step 6 before being used.

(b)

The student uses exactly $2.136\text{ g}$ of powdered eggshell and obtains the results shown in Table 1.1.

Table 1.1

titration number	rough	1	2	3
final burette reading / $\text{cm}^3$	16.55	32.85	16.10	32.30
initial burette reading / $\text{cm}^3$	0.00	16.55	0.10	16.10
titre / $\text{cm}^3$

(i)

Complete Table 1.1. Calculate the mean titre.

(ii)

Calculate the amount, in mol, of unreacted $\text{HCl}(\text{aq})$ in the solution prepared in step 7. Show your working.

(iii)

Calculate the amount, in mol, of $\text{CaCO}_3$ that reacts with the excess of acid. Use your answer to calculate the percentage by mass of $\text{CaCO}_3$ in the eggshell. Show your working.

(c)

Name a suitable piece of apparatus which could be used to transfer $25.00\text{ cm}^3$ of solution in step 8.

(d)

In step 4, a conical flask is weighed using a balance accurate to three decimal places and the mass recorded. The eggshell is placed in the conical flask and the mass increases by $2.136\text{ g}$ .

Calculate the percentage error in measuring the mass of this eggshell.
Show your working.

(e)

State the effect on the percentage by mass if the eggshell is not completely dried in step 2. Explain your answer.

(f)

The student repeats the method using the same apparatus, but decides to use $0.100\text{ mol dm}^{-3}$ $\text{NaOH}(\text{aq})$ to reduce the risk of corrosion or damage to eyes.

Explain how this introduces a weakness to the experimental procedure.

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