Chemistry 9701/31 — May/June 2022

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

questions

marks

120

minutes

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

Q1Manipulation, Measurement and ObservationPresentation of Data and ObservationsAnalysis, Conclusions and EvaluationFree sample

In this experiment you will identify a straight-chain carboxylic acid by titrating an aqueous solution of this acid with aqueous sodium hydroxide. 1 mole of the carboxylic acid reacts with 1 mole of sodium hydroxide. The carboxylic acid contains C, H and O atoms only and has no C=C bonds.

FA 1 is an aqueous solution of the carboxylic acid, containing $10.50\text{ g dm}^{-3}$ .
FA 2 is $0.110\text{ mol dm}^{-3}$ sodium hydroxide, NaOH.
FA 3 is thymolphthalein indicator.

(a)

Method

● Fill the burette with FA 2.
● Pipette $25.0\text{ cm}^3$ of FA 1 into a conical flask.
● Add approximately 8 drops of FA 3.
● 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 sure any recorded results show the precision of your practical work.
● Record in a suitable form below all your burette readings and the volume of FA 2 added in each accurate titration.

(b)

From your accurate titration results, calculate a suitable mean value to use in your calculations. Show clearly how you obtain the mean value.

$25.0\text{ cm}^3$ of FA 1 required .............................. $\text{cm}^3$ of FA 2.

(c)

Calculations

(i)

Calculate the amount, in mol, of sodium hydroxide present in the volume of FA 2 calculated in (b).

amount of NaOH = .............................. mol

(ii)

Use your answer to (c)(i) and the information on page 2 to calculate the relative formula mass of the carboxylic acid in FA 1.

$M_r$ of carboxylic acid = ..............................

(iii)

Identify the carboxylic acid in FA 1. Draw its skeletal formula.

skeletal formula

name of acid .......................................................................................................................

(d)

A student carries out a similar titration to the titration you carried out in (a). The only difference is that a solution of aminoethanoic acid, $\text{NH}_2\text{CH}_2\text{CO}_2\text{H}$ , containing $10.50\text{ g dm}^{-3}$ is used instead of the acid in FA 1.

(i)

Construct an equation for the reaction taking place in the student’s titration. Include state symbols.

(ii)

State whether the student’s titre will be larger or smaller than your titre. Explain your answer.

The student’s titre will be .............................. than mine.

explanation .........................................................................................................................

.............................................................................................................................................

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2 more questions

Q2Presentation of Data and Observations · Manipulation, Measurement and Observation · Analysis, Conclusions and Evaluation11M
Q3Qualitative Analysis15M

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