2025 JC2 Prelims H2 Chem Paper 4 QP and Ans_TJC

Review of H2 Paper 4 1 Determination of the percentage by mass of iron in the wire Iron wire contains impurities. In this experiment, you will investigate the percentage by mass of iron in a sample of iron wire. A sample of iron wire is reacted with an excess of sulfuric acid to produce a solution of iron(II) sulfate. You will titrate the solution of iron( II) sulfate with potassium manganate( VII) of known concentration to determine the amount of iron(II) ions present and hence percentage by mass of iron in the wire. You may assume that impurities do not react with potassium manganate(VII). Iron(II) ions react with manganate(VII) ions according to the equation shown. 5Fe2+ + MnO4– + 8H+ → 5Fe3+ + Mn2+ + 4H2O FA 1 is 0.0200 mol dm–3 potassium manganate(VII), KMnO4. FA 2 is a diluted solution of FeSO4 prepared as follows: • 48.9 g of iron wire was reacted with sulfuric acid to make 1.00 dm3 of solution. • 34.00 cm3 of the solution was then made up to 250 cm3 with deionised water. FA 3 is dilute sulfuric acid, H2SO4. (a) (i) Procedure 1. Fill the burette with FA 1. 2. Pipette 25.0 cm3 of FA 2 into a 250 cm3 conical flask. 3. Use a measuring cylinder to add 25 cm3 of FA 3 into the conical flask. 4. Add FA 1 from the burette until the solution in the conical flask turns to a permanent pale pink colour. 5. Record your titration results, to an appropriate level of precision in the space on page 3. 6. Repeat steps 2 to 5 until consistent results are obtained.

Titration results Final Burette Reading / cm3 22.80 22.80 Initial Burette Reading / cm3 0.00 0.00 Volume of FA1 / cm3 22.80 22.80   Correct headers with units [1] All burette readings to 0.05 cm3 [1] Correct calculation of titre volumes i.e. final – initial burette reading and consistent results within 0.10 cm3. [1] [3] (ii) From your titrations, obtain a suitable volume of FA 1 to be used in your calculations. Show clearly how you obtained this volume. Average titre volume = (22.80 + 22.80)/2 = 22.80 cm3 Use (at least two) titre values within 0.20 cm3 to correctly calculate average volume of FA 1. Working must be shown or ticks put next to the two (or more) consistent titres selected. [1] Accuracy [2] 2 marks if difference is  0.20 cm3 1 mark if difference is  0.40 cm3 0 mark if difference is  0.40 cm3 volume of FA 1 = ……………………………………… [3] (b) (i) Calculate the amount of iron(II) ions present in 25.0 cm3 of FA 2. Amount of MnO4– used = (𝟐𝟐.𝟖𝟎)(𝟎.𝟎𝟐𝟎𝟎) (𝟏𝟎𝟎𝟎) = 4.56 × 10−4 mol Amount of iron(II) ions present = 4.56 × 10−4 × 5 = 2.28 × 10−3 mol [1] 𝑎𝑛𝑠 𝑖𝑛 (𝑎)(𝑖𝑖) 1000 × 0.0200 × 5 amount of iron(II) ions = …………………………………… [1]

(ii) Calculate the mass of iron present in 25.0 cm3 of FA 2. [Ar: Fe, 55.8] Mass of iron present = 2.28 × 10−3 × 55.8 = 0.127 g [1] ans in (b)(i) × 55.8 mass of iron = …………………………………… [1] (iii) Calculate the percentage by mass of iron in the sample of iron w