Chemistry 9701/33 — May/June 2020

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

FA 1 is an aqueous solution of a monoprotic organic acid. You will investigate the identity of FA 1 by using a titration method to find its relative molecular mass, $M_r$ .

FA 1 is an aqueous solution containing $6.20\text{ g dm}^{-3}$ of a monoprotic organic acid.
FA 2 is $0.105\text{ mol dm}^{-3}$ sodium hydroxide, NaOH.
thymol blue indicator

(a)

Method

Pipette $25.0\text{ cm}^3$ of FA 1 into a conical flask.
Fill the burette with FA 2.
Add several drops of thymol blue indicator to the conical flask.
Carry out 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 accuracy of your practical work.
Record in a suitable form below all of your burette readings and the volume of FA 2 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 obtained this value.

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

(c)

Calculations

(i)

Calculate the number of moles of sodium hydroxide present in the volume of FA 2 calculated in (b).

\text{moles of NaOH} = \text{.............................. mol}

(ii)

Deduce the number of moles of the organic acid present in $25.0\text{ cm}^3$ of FA 1.

\text{moles of organic acid} = \text{.............................. mol}

Hence calculate the concentration, in $\text{mol dm}^{-3}$ , of the organic acid in FA 1. Show your working.

\text{concentration of the organic acid} = \text{.............................. mol dm}^{-3}

(iii)

Calculate the relative molecular mass, $M_r$ , of the organic acid in FA 1.

M_r \text{ of the organic acid} = \text{..............................}

(iv)

From another experiment it is found that FA 1 contains one of the following.

\text{CH}_3\text{COOH} \quad \text{HCOOH} \quad \text{C}_2\text{H}_4\text{ClCOOH} \quad \text{CH}_2\text{CHCOOH}

NaOH(aq) reacts only with the COOH group in the acid.

Deduce which of these acids is present in FA 1. Explain your answer.

(d)

This method of investigation uses the relative molecular mass of the acid. The relative molecular masses of $\text{C}_2\text{H}_5\text{COOH}$ and $\text{CH}_2\text{CHCOOH}$ are similar so that any inaccuracy in the practical procedure could lead to an incorrect conclusion.

Suggest a chemical test that would enable you to distinguish between $\text{C}_2\text{H}_5\text{COOH}$ and $\text{CH}_2\text{CHCOOH}$ . Include the test and the results expected but do not carry out this test.

(e)

A student is given a solution of another organic acid containing the same concentration, in $\text{mol dm}^{-3}$ , as that used in (a). The student assumes this acid is monoprotic but it is diprotic.

Explain the effect the student’s assumption has on the value of the relative molecular mass that the student calculates.

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Q2Manipulation, Measurement and Observation · Presentation of Data and Observations · Analysis, Conclusions and Evaluation12M
Q3Manipulation, Measurement and Observation · Qualitative Analysis · Analysis, Conclusions and Evaluation13M

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