Chemistry 9701/33 — May/June 2018

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

Acids are defined as substances that can donate hydrogen ions, $\text{H}^+$ , to bases. Monoprotic acids contain one $\text{H}^+$ that can be donated per molecule. Diprotic acids contain two $\text{H}^+$ that can be donated per molecule.

You will determine by a titration method whether acid $\text{Z}$ is monoprotic or diprotic.

FA 1 is a solution containing $6.10 \text{ g dm}^{-3}$ of acid $\text{Z}$ .
FA 2 is $0.105 \text{ mol dm}^{-3}$ aqueous sodium hydroxide, $\text{NaOH}$ .
bromophenol blue indicator

(a)

Method

Pipette $25.0 \text{ cm}^3$ of FA 1 into a conical flask.
Fill a burette with FA 2.
Add several drops of bromophenol 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 for the volume of FA 2 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}

Then deduce the number of moles of $\text{H}^+$ present in $25.0 \text{ cm}^3$ of FA 1.

\text{moles of H}^+ \text{ in } 25.0 \text{ cm}^3 \text{ of FA 1} = .............................. \text{ mol}

(ii)

Calculate the number of moles of $\text{H}^+$ present in $1 \text{ dm}^3$ of FA 1.

\text{moles of H}^+ \text{ in } 1 \text{ dm}^3 \text{ of FA 1} = .............................. \text{ mol}

(iii)

FA 1 contains $6.10 \text{ g dm}^{-3}$ of acid $\text{Z}$ . The relative molecular mass of $\text{Z}$ is 126.

Calculate the number of moles of $\text{Z}$ in $1 \text{ dm}^3$ of FA 1.

\text{moles of Z in } 1 \text{ dm}^3 \text{ of FA 1} = .............................. \text{ mol}

(iv)

Use your answers to (ii) and (iii) to determine whether $\text{Z}$ is a monoprotic or a diprotic acid. Explain your answer.

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2 more questions

Q2Presentation of Data and Observations · Analysis, Conclusions and Evaluation15M
Q3Qualitative Analysis · Manipulation, Measurement and Observation13M

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