Chemistry 9701/35 — May/June 2014

Cambridge AS Level · Advanced Practical Skills · worked solutions for every part, with the mark scheme

questions

marks

120

minutes

Topics Manipulation, Measurement and Observation · Analysis, Conclusions and Evaluation · Presentation of Data and Observations · Qualitative Analysis

Q1Manipulation, Measurement and ObservationPresentation of Data and ObservationsAnalysis, Conclusions and EvaluationQualitative AnalysisFree sample

Limewater is a saturated solution of calcium hydroxide. A sample of limewater was diluted to produce solution FA 1. You are to determine the concentration of calcium hydroxide, $\text{Ca(OH)}_2$ , in FA 1 by titration with hydrochloric acid, $\text{HCl}$ , which you will first dilute. The equation for the reaction between calcium hydroxide and hydrochloric acid is given below.

\text{Ca(OH)}_2(\text{aq}) + 2\text{HCl}(\text{aq}) \rightarrow \text{CaCl}_2(\text{aq}) + 2\text{H}_2\text{O}(\text{l})

FA 1 is aqueous calcium hydroxide, $\text{Ca(OH)}_2$ .
FA 2 is $0.500\text{ mol dm}^{-3}$ hydrochloric acid, $\text{HCl}$ .
methyl orange indicator

(a)

Method

Read through the method before starting any practical work.

(i)

Dilution

Fill the burette with FA 2.
Run between $24.00$ and $26.00\text{ cm}^3$ of FA 2 into the $250\text{ cm}^3$ volumetric flask.
Record your burette readings and the volume used in the space below.
Make up the solution to $250\text{ cm}^3$ with distilled water and shake the flask to ensure thorough mixing. This solution is FA 3.

volume of $0.500\text{ mol dm}^{-3}$ $\text{HCl}$ used = ............................ $\text{cm}^3$

(ii)

Titration

Empty and rinse the burette.
Fill the burette with FA 3.
Pipette $25.0\text{ cm}^3$ of FA 1 into a conical flask.
Add a few drops of methyl orange indicator.
Perform a rough titration and record your burette readings in the space below.

The rough titre is ............................ $\text{cm}^3$ .

Carry out as many accurate titrations as you think necessary to obtain consistent results.
Make certain any recorded results show the precision of your practical work.
Record, in a suitable form below, all of your burette readings and the volume of FA 3 added in each accurate titration.

(b)

From your accurate titration results, obtain a suitable value to be used in your calculations. Show clearly how you have obtained this value.

$25.0\text{ cm}^3$ of FA 1 required ..................... $\text{cm}^3$ of FA 3.

(c)

Calculations

Show your working and appropriate significant figures in the final answer to each step of your calculations.

(i)

Use your volume of FA 2 from (a)(i) to calculate the concentration of FA 3.

concentration of FA 3 = ............................ $\text{mol dm}^{-3}$

(ii)

Use your answer to (c)(i) and the value calculated in (b) to calculate the number of moles of hydrochloric acid used to neutralise $25.0\text{ cm}^3$ of FA 1.

moles of $\text{HCl}$ = ............................ $\text{mol}$

(iii)

Use your answer to (c)(ii) to calculate the number of moles of calcium hydroxide in $25.0\text{ cm}^3$ of FA 1.

moles of $\text{Ca(OH)}_2$ = ............................ $\text{mol}$

(iv)

Use your answer to (c)(iii) to calculate the concentration, in $\text{mol dm}^{-3}$ , of calcium hydroxide in FA 1.

concentration of $\text{Ca(OH)}_2$ = ............................ $\text{mol dm}^{-3}$

(d)

(i)

The maximum error in a single burette reading is $\pm0.05\text{ cm}^3$ .
When making up the diluted acid, FA 3, a student recorded that $24.80\text{ cm}^3$ of FA 2 was used.
What are the smallest and largest possible volumes of acid that were run into the volumetric flask?

smallest volume used = ................. $\text{cm}^3$

largest volume used = ................. $\text{cm}^3$

(ii)

If the actual volume of acid added had been less than $24.80\text{ cm}^3$ , how would the value the student calculated for the concentration of $\text{Ca(OH)}_2$ in (c)(iv) compare to the true value? Explain your answer.

(e)

If a sample of limewater is left open to the air, the concentration of calcium hydroxide decreases. Explain why.

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2 more questions

Q2Manipulation, Measurement and Observation · Presentation of Data and Observations · Analysis, Conclusions and Evaluation10M
Q3Qualitative Analysis · Manipulation, Measurement and Observation · Analysis, Conclusions and Evaluation

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