Chemistry 9701/32 — 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 · Presentation of Data and Observations · Analysis, Conclusions and Evaluation · Qualitative Analysis

Q1Manipulation, Measurement and ObservationPresentation of Data and ObservationsAnalysis, Conclusions and EvaluationFree sample

In this question you will determine the percentage purity of a sample of contaminated sodium carbonate. FB 1 is a solution of the contaminated sodium carbonate. You will first dilute FB 1 and then titrate the diluted solution using hydrochloric acid.

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

FB 1 was prepared by dissolving $125\text{ g}$ of contaminated sodium carbonate, $\text{Na}_2\text{CO}_3$ , in distilled water and making the solution up to $1\text{ dm}^3$ .
FB 2 is $0.100\text{ mol dm}^{-3}$ hydrochloric acid, $\text{HCl}$ .
methyl orange indicator

(a)

Method

Dilution

Fill the burette with FB 1.
Run between $13.00$ and $13.50\text{ cm}^3$ of FB 1 into the $250\text{ cm}^3$ volumetric (graduated) flask. Record the volume in the space below.

$\text{volume of FB 1} = \text{...................... cm}^3$

Fill the volumetric flask to the line with distilled water. Stopper the flask and shake it to ensure thorough mixing.
Label this flask FB 3.

Titration

Rinse the burette thoroughly with distilled water and then with a little of solution FB 2.
Fill the burette with FB 2.
Use the pipette to transfer $25.0\text{ cm}^3$ of FB 3 into a conical flask.
Add 5-10 drops of methyl orange indicator.
Perform a rough titration and record your burette readings in the space below.

$\text{The rough titre is ...................... cm}^3.$

Do 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 FB 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\text{ of FB 3 required .......................... cm}^3\text{ of FB 2.}

(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 hydrochloric acid present in the volume of FB 2 calculated in (b).

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

(ii)

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

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

(iii)

Calculate the concentration, in $\text{mol dm}^{-3}$ , of sodium carbonate in FB 3.

$\text{concentration of Na}_2\text{CO}_3\text{ in FB 3} = \text{............................. mol dm}^{-3}$

(iv)

Calculate the concentration, in $\text{mol dm}^{-3}$ , of sodium carbonate in FB 1.

$\text{concentration of Na}_2\text{CO}_3\text{ in FB 1} = \text{............................. mol dm}^{-3}$

(v)

Calculate the percentage purity by mass of the sodium carbonate in the contaminated sample used to prepare solution FB 1.
( $A_r$ : C, 12.0; O, 16.0; Na, 23.0)

$\text{percentage purity by mass} = \text{............................. \%}$

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2 more questions

Q2Presentation of Data and Observations · Manipulation, Measurement and Observation · Analysis, Conclusions and Evaluation11M
Q3Qualitative Analysis15M

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