Chemistry 9701/35 — October/November 2014

Cambridge AS Level · Advanced Practical Skills · worked solutions for every part, with the mark scheme

questions

marks

120

minutes

Topics Analysis, Conclusions and Evaluation · Manipulation, Measurement and Observation · Presentation of Data and Observations · Qualitative Analysis

Q1Manipulation, Measurement and ObservationPresentation of Data and ObservationsQualitative AnalysisAnalysis, Conclusions and EvaluationFree sample

In this experiment you are to determine $x$ in the formula of hydrated sodium carbonate, $\text{Na}_2\text{CO}_3 \cdot x\text{H}_2\text{O}$ , by titration.

FA 1 is aqueous sodium carbonate, $\text{Na}_2\text{CO}_3$ , containing $150.0\text{ g}$ of solid $\text{Na}_2\text{CO}_3 \cdot x\text{H}_2\text{O}$ in each $\text{dm}^3$ of solution.
FA 2 is $0.110\text{ mol dm}^{-3}$ nitric acid, $\text{HNO}_3$ .
methyl orange indicator

(a)

Method

Diluting the solution of FA 1

Pipette $25.0\text{ cm}^3$ of FA 1 into the $250\text{ cm}^3$ volumetric (graduated) flask.
Make the solution up to the mark using distilled water.
Shake the flask thoroughly to mix the solution before using it for your titrations.
This solution of sodium carbonate is FA 3.
Rinse the pipette with distilled water.

Titration

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

Q1_titration_table

(b)

From your accurate titration results, obtain a suitable value to be used in your calculations. Show clearly how you have obtained this value.

volume of FA 2 = ................ $\text{cm}^3$

(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 nitric acid in the volume of FA 2 calculated in (b).

moles of $\text{HNO}_3$ = ..................... mol

(ii)

Complete the equation below and include the missing state symbols.

\text{Na}_2\text{CO}_3(\text{aq}) + \text{HNO}_3(\text{aq}) \rightarrow \text{NaNO}_3 + \text{CO}_2 + \text{H}_2\text{O}

(iii)

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

moles of $\text{Na}_2\text{CO}_3$ in $25.0\text{ cm}^3$ of FA 3 = ....................... mol

(iv)

Calculate the number of moles of sodium carbonate in $250.0\text{ cm}^3$ of solution FA 3.

moles of $\text{Na}_2\text{CO}_3$ in $250.0\text{ cm}^3$ of FA 3 = ....................... mol

(v)

Use the information on page 2 and your answer to (iv) to calculate the relative formula mass, $M_r$ , of hydrated sodium carbonate, $\text{Na}_2\text{CO}_3 \cdot x\text{H}_2\text{O}$ .

$M_r$ of $\text{Na}_2\text{CO}_3 \cdot x\text{H}_2\text{O}$ = .................................

(vi)

Calculate the value of $x$ in $\text{Na}_2\text{CO}_3 \cdot x\text{H}_2\text{O}$ . Give your answer to the nearest whole number.
[ $A_r$ : H, 1.0; C, 12.0; O, 16.0; Na, 23.0]

$x$ = ......................

(d)

(i)

State the maximum error in any single reading of the burette.

maximum error = .................. $\text{cm}^3$

(ii)

Calculate the maximum percentage error in the volume of FA 2 in your first accurate titre.

maximum percentage error = .......................%

The rest of this paper

2 more questions

Q2Presentation of Data and Observations · Analysis, Conclusions and Evaluation13M
Q3Qualitative Analysis · Analysis, Conclusions and Evaluation · Manipulation, Measurement and Observation12M

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