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9701/35

Chemistry 9701/35 — October/November 2017

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

3
questions
40
marks
120
minutes

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

Q1Manipulation, Measurement and ObservationPresentation of Data and ObservationsAnalysis, Conclusions and EvaluationFree 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 mol dm−30.0197\text{ mol dm}^{-3} solution of the iodine-containing compound.
FA 2 is dilute sulfuric acid, H2SO4\text{H}_2\text{SO}_4.
FA 3 is aqueous potassium iodide, KI.
FA 4 is 0.105 mol dm−30.105\text{ mol dm}^{-3} sodium thiosulfate, Na2S2O3\text{Na}_2\text{S}_2\text{O}_3.
starch indicator

FA 1 reacts with excess acidified potassium iodide to produce iodine, I2\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 cm325.0\text{ cm}^3 of FA 1 into a conical flask.
  • Using the measuring cylinder, add 10 cm310\text{ cm}^3 of FA 2 to the same conical flask.
  • Using the same measuring cylinder, add 20 cm320\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 ............................. cm3\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.
7M
(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 ............................. cm3\text{cm}^3 of FA 4.

1M
(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 Na2S2O3=............................ mol\text{moles of Na}_2\text{S}_2\text{O}_3 = \text{............................ mol}
(ii)

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

I2(aq)+2Na2S2O3(aq)→Na2S4O6(aq)+2NaI(aq)\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 I2=............................ mol\text{moles of I}_2 = \text{............................ mol}
(iii)

Use the information on page 2 to calculate the number of moles of iodine-containing compound in the 25 cm325\text{ cm}^3 of FA 1 used in each titration.

moles of iodine-containing compound in 25 cm3 FA 1=............................ mol\text{moles of iodine-containing compound in } 25\text{ cm}^3\text{ FA 1} = \text{............................ 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 I2=............................ mol\text{moles of I}_2 = \text{............................ mol}
(v)

The anion in FA 1 is IOx−\text{IO}_x^- where xx 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 xx in the formula IOx−\text{IO}_x^-.

IO…−+…I−+…H+→…I2+…H2O\text{IO}_{\dots}^- + \dots\text{I}^- + \dots\text{H}^+ \rightarrow \dots\text{I}_2 + \dots\text{H}_2\text{O} x=............................x = \text{............................}
(vi)

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

oxidation state of iodine=............................\text{oxidation state of iodine} = \text{............................}
6M

The rest of this paper

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