Chemistry 9701/32 — May/June 2017

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

HA is an organic acid where $\text{A}^-$ is the anion. You will determine the relative formula mass, $M_r$ , of HA by titration with sodium hydroxide of known concentration and so identify the anion, $\text{A}^-$ . The equation for the reaction is shown.

\text{HA(aq)} + \text{NaOH(aq)} \rightarrow \text{NaA(aq)} + \text{H}_2\text{O(l)}

FB 1 is a solution of organic acid, HA, containing $12.60\text{ g dm}^{-3}$ .
FB 2 is $0.100\text{ mol dm}^{-3}$ sodium hydroxide, NaOH.
thymol blue indicator

(a)

Method

Fill the burette with FB 1.
Pipette $25.0\text{ cm}^3$ of FB 2 into a conical flask.
Add approximately 10 drops of thymol blue indicator. This indicator is blue in alkaline solutions and yellow in acidic solutions.
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 that 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 FB 1 added in each accurate titration.

Fig. 1.1

(b)

From your accurate titration results, obtain a suitable value for the volume of FB 1 to be used in your calculations. Show clearly how you obtained this value.

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

(c)

Calculations

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

(i)

Calculate the number of moles of sodium hydroxide present in $25.0\text{ cm}^3$ of FB 2 pipetted into the conical flask.

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

(ii)

Use your answers to (b) and (c)(i) to determine the concentration of the organic acid HA, in FB 1, in $\text{mol dm}^{-3}$ .

concentration of HA in FB 1 = ............................ $\text{mol dm}^{-3}$

(iii)

Use your answer to (ii) and the information given on page 2 to determine the relative formula mass, $M_r$ , of the organic acid, HA.

$M_r$ of HA = ............................

(iv)

The organic acid was known to have one of the following structural formulae.

\text{CH}_3\text{COOH} \quad \text{CH}_2\text{ClCOOH} \quad \text{CHCl}_2\text{COOH} \quad \text{CCl}_3\text{COOH}

Use your answer to (iii) and the Periodic Table on page 16 to identify the anion, $\text{A}^-$ .

anion, $\text{A}^-$ = ............................

(d)

A student carried out the same procedure accurately but was supplied with a solution of less concentrated sodium hydroxide by mistake.

(i)

What effect would this have on the calculated value of the relative formula mass, $M_r$ ? Explain your answer.

(ii)

Explain how this would affect the identification of the acid.

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2 more questions

Q2Manipulation, Measurement and Observation · Presentation of Data and Observations · Analysis, Conclusions and Evaluation12M
Q3Qualitative Analysis6M

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