Chemistry 9701/31 — 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 ObservationsFree sample

In this experiment you will use a solution of sodium carbonate, $\text{Na}_2\text{CO}_3$ , to determine the concentration of a solution of hydrochloric acid, $\text{HCl}$ , by carrying out a titration.

\text{Na}_2\text{CO}_3(\text{aq}) + 2\text{HCl}(\text{aq}) \rightarrow 2\text{NaCl}(\text{aq}) + \text{CO}_2(\text{g}) + \text{H}_2\text{O}(\text{l})

FA 1 is a solution of sodium carbonate containing $1.30\text{ g}$ $\text{Na}_2\text{CO}_3$ in each $250\text{ cm}^3$ .
FA 2 is hydrochloric acid, $\text{HCl}$ .
methyl orange indicator

(a)

Method

Fill a burette with FA 2.
Use the pipette to transfer $25.0\text{ cm}^3$ of FA 1 into a conical flask.
Add a few drops of methyl orange indicator.
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 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 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)

Give your answer to (ii), (iii) and (iv) to an appropriate number of significant figures.

(ii)

Calculate the number of moles of sodium carbonate present in $25.0\text{ cm}^3$ of FA 1.

moles of $\text{Na}_2\text{CO}_3$ = ............................. mol

(iii)

Calculate the number of moles of hydrochloric acid that reacted with the number of moles of sodium carbonate you calculated in (ii).

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

(iv)

Use your answers to (b) and (c)(iii) to calculate the concentration of hydrochloric acid in FA 2.

concentration of $\text{HCl}$ in FA 2 = ............................. $\text{mol dm}^{-3}$

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2 more questions

Q2Manipulation, Measurement and Observation · Presentation of Data and Observations · Analysis, Conclusions and Evaluation11M
Q3Qualitative Analysis · Manipulation, Measurement and Observation · Analysis, Conclusions and Evaluation17M

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