Chemistry 9701/31 — October/November 2017

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

questions

marks

120

minutes

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

Q1Manipulation, Measurement and ObservationPresentation of Data and ObservationsAnalysis, Conclusions and EvaluationQualitative AnalysisFree sample

In this experiment you will determine the oxidation number of iodine in one of its compounds by titration.

FA 1 is a $0.0197\text{ mol dm}^{-3}$ solution of the iodine-containing compound.
FA 2 is dilute sulfuric acid, $\text{H}_2\text{SO}_4$ .
FA 3 is aqueous potassium iodide, $\text{KI}$ .
FA 4 is $0.105\text{ mol dm}^{-3}$ sodium thiosulfate, $\text{Na}_2\text{S}_2\text{O}_3$ .
starch indicator

FA 1 reacts with excess acidified potassium iodide to produce iodine, $\text{I}_2$ . This iodine is then titrated with aqueous sodium thiosulfate using starch indicator.

(a)

Method

Fill the burette with FA 4.
Pipette $25.0\text{ cm}^3$ of FA 1 into a conical flask.
Using the measuring cylinder, add $10\text{ cm}^3$ of FA 2 to the same conical flask.
Using the same measuring cylinder, add $20\text{ cm}^3$ of FA 3 to the mixture in the conical flask. The mixture will now be a red-brown colour, due to iodine produced.
Carry out a rough titration by adding FA 4 from the burette until the mixture becomes light brown.
Then add 10 drops of starch indicator. The mixture will change to a dark blue colour.
Continue titrating until the mixture becomes colourless. This is the end-point.

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 4 added in each accurate titration.

Q1_table

(b)

From your accurate titration results, obtain a suitable value for the volume of FA 4 to be used in your calculations. Show clearly how you obtained this value.

The iodine produced required ............................. $\text{cm}^3$ of FA 4.

(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 thiosulfate in the volume of FA 4 calculated in (b).

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

(ii)

The equation for the reaction of iodine with sodium thiosulfate is shown.

\text{I}_2(\text{aq}) + 2\text{Na}_2\text{S}_2\text{O}_3(\text{aq}) \rightarrow \text{Na}_2\text{S}_4\text{O}_6(\text{aq}) + 2\text{NaI}(\text{aq})

Calculate the number of moles of iodine that reacted with the sodium thiosulfate calculated in (i).

moles of $\text{I}_2$ = ............................ mol

(iii)

Use the information on page 2 to calculate the number of moles of iodine-containing compound in the $25\text{ cm}^3$ of FA 1 used in each titration.

moles of iodine-containing compound in $25\text{ cm}^3$ FA 1 = ............................ mol

(iv)

Use your answers to (ii) and (iii) to calculate the number of moles of iodine produced when 1 mole of the iodine-containing compound in FA 1 reacts with excess FA 3. Give your answer as an integer.

moles of $\text{I}_2$ = ............................ mol

(v)

The anion in FA 1 is $\text{IO}_x^-$ where $x$ is the number of oxygen atoms present in the formula.

Use your answer to (iv) to balance the ionic equation for the reaction between FA 1 and FA 3 under acidic conditions.
Hence deduce the value of $x$ in the formula $\text{IO}_x^-$ .

\text{IO}_x^- + \dots\text{I}^- + \dots\text{H}^+ \rightarrow \dots\text{I}_2 + \dots\text{H}_2\text{O}

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

(vi)

Calculate the oxidation state of iodine in FA 1.
(If you were unable to calculate $x$ in part (v), assume that $x = 4$ .)

oxidation state of iodine = ............................

The rest of this paper

2 more questions

Q2Manipulation, Measurement and Observation · Qualitative Analysis · Presentation of Data and Observations · Analysis, Conclusions and Evaluation14M
Q3Qualitative Analysis · Presentation of Data and Observations · Analysis, Conclusions and Evaluation12M

Loading the full paper…