3.2.5.5 Variable oxidation states (A-level only)

A 0.0720 g sample of reducing agent R was dissolved in water and acidified with an excess of dilute H₂SO₄. The resulting solution was found to react with exactly 18.0 cm³ of a 0.0200 mol dm⁻³ solution of KMnO₄.

In this reaction, 5 mol of R react with 3 mol of KMnO₄. The M_r of R is

• 120

• 167

• 240

• 333

The vanadium does not have an oxidation state of +3 in

• [V(H₂O)₆]³⁺

• [V(C₂O₄)₃]³⁻

• [V(OH)₃(H₂O)₃]

• [VCl₄]³⁻

Use your knowledge of the chemistry of transition metals to predict which of the following will convert [Mn(H₂O)₆]²⁺ into MnO₄^2-

• an acid and a reducing agent

• an acid and an oxidising agent

• an alkali and a reducing agent

• an alkali and an oxidising agent

Which one of the following would not reduce an acidified aqueous solution of potassium dichromate(VI)?

• CH₃COOH

• Zn

• CH₃CHO

• Fe²⁺(aq)

Which one of the following statements about the reaction below is false?

[Cu(H₂O)₆]²⁺ + EDTA⁴⁻ ⇌ [Cu(EDTA)]^2- + 6H₂O

• [Cu(EDTA)]²⁻ is a more stable complex than [Cu(H₂O)₆]²⁺

• Both [Cu(H₂O)₆]²⁺ and [Cu(EDTA)]²⁻ are octahedral complexes.

• There is an increase in entropy when the reaction occurs.

• There is a redox reaction.

Which one of the following can act as an oxidising agent but not as a reducing agent?

• CH₃CHO

• Fe²⁺

• I⁻

• MnO₄^-

Aqueous C₂O₄^2- ions react with MnO₄^- ions in acidic solution according to the equation

5 C₂O₄^2- + 2MnO₄^- + 16H⁺ → 2Mn²⁺ + 10CO₂ + 8H₂O

Under the same conditions Fe²⁺ ions also react with 2MnO₄^- ions. How many moles of 2MnO₄^- ions are required to react exactly with one mole of Fe(C₂O₄).2H₂O?

• 0.4

• 0.6

• 2.5

• 7.5

The percentage of iron in a sample of impure iron(II) sulphate crystals can be determined by titrating solutions, made from separate weighed samples acidified with dilute sulphuric acid, against a standard solution of potassium manganate(VII).

Which one of the following statements explains why dilute hydrochloric acid is unsuitable for use in this titration?

• HCl will oxidise Fe²⁺ to Fe³⁺

• Cl⁻ will reduce Fe³⁺ to Fe²⁺

• Cl⁻ will reduce MnO₄^-

• HCl is a strong acid

The percentage of iron in a sample of impure iron(II) sulphate crystals can be determined by titrating solutions, made from separate weighed samples acidified with dilute sulphuric acid, against a standard solution of potassium manganate(VII).

Which one of the following would lead to an inaccurate result?

• transferring the weighed sample of iron(II) sulphate into a wet conical flask

• failing to measure accurately the volume of water used to dissolve each weighed sample of iron(II) sulphate

• transferring the standard solution of potassium manganate(VII) from its original container to the burette using a wet beaker

• failing to measure accurately the volume of dilute sulphuric acid added to the mixture before titration

The percentage of iron in a sample of impure iron(II) sulphate crystals can be determined by titrating solutions, made from separate weighed samples acidified with dilute sulphuric acid, against a standard solution of potassium manganate(VII).

Which one of the following would lead to the greatest error in the calculation of the percentage of iron(II) in the sample?

• an error of 0.005 g made when weighing out a sample of mass 0.987 g

• an end-point error of 0.1 cm³ in 25.0 cm³

• an error of 5 cm³ when measuring out 25.0 cm³ of dilute sulphuric acid

• using the average of the titration values 25.4, 25.7 and 25.9 when the correct value is 25.5 cm³