Chemistry 9701/33 — May/June 2015

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 EvaluationFree sample

HA is an organic acid. Solution FA 1 was prepared by dissolving 13.1 g of solid HA in each dm $^3$ of solution. You are to determine the relative molecular mass, $M_r$ , of HA by titration with aqueous sodium hydroxide. The equation for the reaction between HA and sodium hydroxide is given below.

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

FA 1 is a solution containing $13.1 \text{ g dm}^{-3}$ of organic acid, HA.
FA 2 is $0.100 \text{ mol dm}^{-3}$ sodium hydroxide, NaOH.
bromothymol blue indicator

(a)

Method

Fill the burette with FA 1.
Use the pipette to transfer $25.0 \text{ cm}^3$ of FA 2 into a conical flask.
Add a few drops of bromothymol blue indicator. This indicator is blue in alkali and yellow in acid solutions.
Perform a rough titration and record your burette readings in the space below.

The rough titre is ...................... 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 1 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 2 required ...................... cm $^3$ of FA 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 the volume of FA 2 pipetted into the conical flask.

moles of NaOH = .......................... mol

(ii)

Use your answer to (c)(i) and the equation on page 2 to determine the number of moles of organic acid, HA, used to neutralise $25.0 \text{ cm}^3$ of FA 2.

moles of HA = .......................... mol

(iii)

Use your answers to (b) and (c)(ii) to calculate the number of moles of HA in $1 \text{ dm}^3$ of FA 1.

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

(iv)

Calculate the relative molecular mass, $M_r$ , of the organic acid, HA.

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

(d)

A student carrying out this method correctly with the same concentration of reactants obtained a titre of $28.30 \text{ cm}^3$ . Would this give a larger or smaller value of $M_r$ than yours?
Explain your answer.

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2 more questions

Q2Presentation of Data and Observations · Manipulation, Measurement and Observation · Analysis, Conclusions and Evaluation15M
Q3Manipulation, Measurement and Observation · Qualitative Analysis · Analysis, Conclusions and Evaluation3M

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