May/June 2026 Paper 52

2026 · 2 questions · 21 parts · 30 marks

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1(a)(i)1(a)(ii)1(b)(i)1(b)(ii)1(b)(iii)1(b)(iv)1(c)(i)1(c)(ii)1(c)(iii)1(c)(iv)1(c)(v)

–/16 mks

2(a)2(b)2(c)2(d)2(e)(i)2(e)(ii)2(e)(iii)2(e)(iv)2(e)(v)2(e)(vi)

–/14 mks

Context for question 1

Under certain conditions, iodine forms a series of polyiodide anions with the formula

I_{2 n + 1}^{-}

, where

n

is an integer.

This question is about the preparation of a polyiodide salt,

(C H_{3})_{4} N I_{2 n + 1}

. After preparing the polyiodide salt, the value of

n

(C H_{3})_{4} N I_{2 n + 1}

is found by titration.

(C H_{3})_{4} N I_{2 n + 1}

is formed when tetramethylammonium iodide,

(C H_{3})_{4} N I

, reacts with methanolic iodine solution,

I_{2}

(meth).

(C H_{3})_{4} N I + n I_{2} \to (C H_{3})_{4} N I_{2 n + 1}

Methanolic iodine solution,

I_{2}

(meth), is prepared by dissolving solid

I_{2}

in methanol.

1(a)(i)Concentration Solutions Titration CalculationsEasy1 mark

Calculate the mass, in g, of

I_{2}

(s) needed to prepare 100.0 cm

^{3}

of 0.300 mol dm

^{- 3}

I_{2}

(meth).

mass =

1(a)(ii)Volumetric Techniques Titration PracticeMedium3 marks

Describe the steps taken to prepare 100.0 cm

^{3}

of 0.300 mol dm

^{- 3}

I_{2}

(meth), starting from the mass of

I_{2}

(s) calculated in 1(a)(i) in a 50 cm

^{3}

beaker. Give the name and capacity of any apparatus used. Write your answer using a series of numbered steps.

Answer

0 words

Context for 1(b)

The following steps are carried out.

1
Add 0.250 g of $(C H_{3})_{4} N I$ (s) to a dry conical flask. Add 10 cm $^{3}$ of the $I_{2}$ (meth) solution prepared in 1(a).
2
Carefully warm the conical flask in a water bath until no solid remains.
3
Allow to cool for about 30 minutes, swirling occasionally. Crystals of the product, $(C H_{3})_{4} N I_{2 n + 1}$ , form.
4
Filter the mixture.
5
Wash the crystals with cold methanol.
6
Transfer the crystals to an evaporating basin and leave to dry.
7
Determine the mass of dry crystals obtained.

1(b)(i)Practical Techniques SafetyEasy1 mark

Suggest why the conical flask is not heated directly with a Bunsen burner in step 2.

1(b)(ii)Separation Purification Organic PreparationEasy1 mark

Suggest why the crystals are washed in step 5.

1(b)(iii)Practical Techniques SafetyEasy1 mark

Suggest how to ensure the crystals are completely dry in step 6.

1(b)(iv)Tables Graphs Data AnalysisMedium2 marks

Describe how the mass of dry crystals obtained can be determined in step 7. Include a results table, with appropriate headings, for recording any measurements.

Design your results table (rows and units)

0 words

Describe how the mass of dry crystals is determined

0 words

Context for 1(c)

0.472 g of

(C H_{3})_{4} N I_{2 n + 1}

crystals are dissolved in 100.0 cm

^{3}

of a solution of ethanolic potassium iodide, KI(eth). This is solution A.

The

I_{2 n + 1}^{-}

ion in

(C H_{3})_{4} N I_{2 n + 1}

is composed of

n I_{2}

molecules and one

I^{-}

ion.

I_{2}

molecules in

I_{2 n + 1}^{-}

ions react with aqueous thiosulfate ions,

S_{2} O_{3}^{2 -}

(aq).

I_{2} + 2 S_{2} O_{3}^{2 -} \to S_{4} O_{6}^{2 -} + 2 I^{-}

10.00 cm

^{3}

portions of solution A are titrated against 0.0200 mol dm

^{- 3}

aqueous sodium thiosulfate,

N a_{2} S_{2} O_{3}

(aq), to determine the amount of

I_{2}

present in the

(C H_{3})_{4} N I_{2 n + 1}

. The titration results are shown in Table 1.1.

Table 1.1

	rough titration	titration 1	titration 2	titration 3
final burette reading / cm $^{3}$	14.05	27.40	41.00	13.75
initial burette reading / cm $^{3}$	0.10	14.05	27.40	0.50
titre / cm $^{3}$	13.95	13.35	13.60	13.25

1(c)(i)Volumetric Techniques Titration PracticeEasy1 mark

State what the burette is rinsed with immediately before filling it with 0.0200 mol dm

^{- 3}

N a_{2} S_{2} O_{3}

(aq) for use in the titrations.

1(c)(ii)Uncertainty Errors EvaluationEasy1 mark

Calculate the percentage error in the measurement of the titre for titration 1.

percentage error =

1(c)(iii)Volumetric Techniques Titration PracticeEasy1 mark

Use data from Table 1.1 to calculate a suitable mean titre to use in the calculations.

mean titre =

1(c)(iv)Concentration Solutions Titration CalculationsMedium3 marks

Calculate the mass, in g, of

I_{2}

present in 100.0 cm

^{3}

of solution A.

mass =

0 words

1(c)(v)Empirical Molecular FormulaeMedium1 mark

0.472 g of

(C H_{3})_{4} N I_{2 n + 1}

crystals were used to make solution A in 1(c). Use this information, the equation shown and your answer to 1(c)(iv) to determine the value of

n

(C H_{3})_{4} N I_{2 n + 1}

(C H_{3})_{4} N I + n I_{2} \to (C H_{3})_{4} N I_{2 n + 1}

n =

0 words