Edexcel A-Level Chemistry: Amount of Substance & Redox

The concentration of an oxidising agent in solution can be found indirectly using an iodine-thiosulfate titration. The oxidising agent is first reacted with an excess of acidified potassium iodide, and the iodine that is liberated is then titrated against a standard sodium thiosulfate solution using starch as an indicator. The two reactions involved can be summarised as:

$\text{oxidising agent} + 2\text{I}^{-} \rightarrow \text{I}_2 + \text{reduced product}$oxidising agent+2I−→I2​+reduced product $\text{I}_2 + 2\text{S}_2\text{O}_3^{2-} \rightarrow 2\text{I}^{-} + \text{S}_4\text{O}_6^{2-}$I2​+2S2​O32−​→2I−+S4​O62−​

Describe and explain how this two-stage titration allows the amount of the oxidising agent to be determined. In your answer you should refer to:

• why an excess of potassium iodide is used and what is observed in the first stage;
• the role of the starch indicator and why it is added only when the solution has faded to a pale straw colour;
• how the mole ratios in the two equations are combined to link the thiosulfate titre to the amount of oxidising agent.

(6 marks)

A technician needs to check the purity of a batch of hydrated iron(II) sulfate crystals, FeSO₄·7H₂O, which may have partly oxidised in storage. A 7.72 g sample of the crystals was dissolved in excess dilute sulfuric acid and the solution made up to exactly 250 cm³ in a volumetric flask. 25.0 cm³ portions of this solution were then titrated against 0.0200 mol dm⁻³ potassium manganate(VII).

The ionic equation for the titration reaction is:

$\text{MnO}_4^{-} + 8\text{H}^{+} + 5\text{Fe}^{2+} \rightarrow \text{Mn}^{2+} + 4\text{H}_2\text{O} + 5\text{Fe}^{3+}$MnO4−​+8H++5Fe2+→Mn2++4H2​O+5Fe3+

Titration	1	2	3	4
Final burette reading / cm³	25.10	49.95	25.05	50.00
Initial burette reading / cm³	0.00	25.00	0.05	25.05
Titre / cm³	25.10	24.95	25.00	24.95

(Relative atomic masses: Fe = 55.8; S = 32.1; O = 16.0; H = 1.0)

(a) Using only the concordant titres, calculate the mean titre of potassium manganate(VII). (1 mark)

(b) Calculate the percentage purity of the FeSO₄·7H₂O in the sample by mass. Show your working clearly. (5 marks)

A newly synthesised solid W is being investigated as a slow-release nitrogen fertiliser. Combustion analysis shows that it contains only carbon, hydrogen, nitrogen and oxygen, in the following proportions by mass.

Element	Carbon	Hydrogen	Nitrogen	Oxygen
Percentage by mass / %	41.4	6.9	24.1	27.6

A mass spectrum of W gives a relative molecular mass of 116.0.

(a) Use the composition data to determine the empirical formula of W. Show your working. (3 marks)

(b) Use the empirical formula and the relative molecular mass to determine the molecular formula of W. (2 marks)

(Relative atomic masses: C = 12.0; H = 1.0; N = 14.0; O = 16.0)

A manufacturer of dry-powder fire suppressants uses sodium hydrogencarbonate, which releases carbon dioxide gas when heated by a flame. On full thermal decomposition it reacts as follows:

$2\text{NaHCO}_3(\text{s}) \rightarrow \text{Na}_2\text{CO}_3(\text{s}) + \text{H}_2\text{O}(\text{l}) + \text{CO}_2(\text{g})$2NaHCO3​(s)→Na2​CO3​(s)+H2​O(l)+CO2​(g)

In a laboratory test, a cartridge containing 42.0 g of sodium hydrogencarbonate is fully decomposed.

(Relative atomic masses: Na = 23.0; C = 12.0; O = 16.0; H = 1.0. The molar volume of a gas at RTP is 24.0 dm³ mol⁻¹.)

(a) Calculate the volume of carbon dioxide, in dm³, released at RTP. (3 marks)

(b) Calculate the mass of sodium carbonate left as the solid residue. (2 marks)

During an iodine-thiosulfate titration, iodine is reduced by thiosulfate ions according to the equation:

$\text{I}_2 + 2\text{S}_2\text{O}_3^{2-} \rightarrow 2\text{I}^{-} + \text{S}_4\text{O}_6^{2-}$I2​+2S2​O32−​→2I−+S4​O62−​

The sulfur atoms in the thiosulfate ion, $\text{S}_2\text{O}_3^{2-}$S2​O32−​, and in the tetrathionate ion, $\text{S}_4\text{O}_6^{2-}$S4​O62−​, can be treated as having a single average oxidation state.

(a) Deduce the average oxidation state of sulfur in the thiosulfate ion, $\text{S}_2\text{O}_3^{2-}$S2​O32−​, and in the tetrathionate ion, $\text{S}_4\text{O}_6^{2-}$S4​O62−​. Show your reasoning. (2 marks)

(b) Using oxidation states, identify the species that is oxidised and the species that is reduced in this reaction, and explain how the oxidation states support your choice. (2 marks)

In acidic solution, dichromate(VI) ions, $\text{Cr}_2\text{O}_7^{2-}$Cr2​O72−​, act as an oxidising agent and are reduced to chromium(III) ions, $\text{Cr}^{3+}$Cr3+.

(a) Deduce the oxidation state of chromium in the dichromate(VI) ion, $\text{Cr}_2\text{O}_7^{2-}$Cr2​O72−​. (1 mark)

(b) Construct the balanced ionic half-equation for the reduction of $\text{Cr}_2\text{O}_7^{2-}$Cr2​O72−​ to $\text{Cr}^{3+}$Cr3+ in acidic solution, including the electrons, $\text{H}^{+}$H+ ions and water. (2 marks)