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The percentage by mass of iron in a steel wire is determined by a student - AQA - A-Level Chemistry - Question 5 - 2019 - Paper 3

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The percentage by mass of iron in a steel wire is determined by a student. The student - reacts 680 mg of the wire with an excess of sulfuric acid, so that all of ... show full transcript

Worked Solution & Example Answer:The percentage by mass of iron in a steel wire is determined by a student - AQA - A-Level Chemistry - Question 5 - 2019 - Paper 3

Step 1

Give the equation for the reaction between iron and sulfuric acid.

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Answer

The reaction between iron and sulfuric acid can be represented as:

extFe+extH2extSO4ightarrowextFeSO4+extH2 ext{Fe} + ext{H}_2 ext{SO}_4 ightarrow ext{FeSO}_4 + ext{H}_2

Step 2

Calculate the mean titre.

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Answer

To calculate the mean titre, we first find the titre values:

  • Titre 1: 22.90 cm³
  • Titre 2: 22.70 cm³
  • Titre 3: 22.60 cm³

Now, we compute the mean:

ext{Mean Titre} = rac{22.90 + 22.70 + 22.60}{3} = rac{68.20}{3} = 22.73 ext{ cm}^3

Step 3

Give the overall ionic equation for the oxidation of Fe²⁺ by manganate(VII) ions, in acidic conditions.

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Answer

The overall ionic equation for the oxidation of Fe²⁺ by manganate(VII) ions in acidic conditions is:

5extFe2++extMnO4−+8extH+ightarrow5extFe3++extMn2++4extH2extO5 ext{Fe}^{2+} + ext{MnO}_4^{-} + 8 ext{H}^{+} ightarrow 5 ext{Fe}^{3+} + ext{Mn}^{2+} + 4 ext{H}_2 ext{O}

Step 4

State the colour change seen at the end point of the titration.

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Answer

The colour change seen at the end point of the titration is from colourless (or pale green) to a faint pink.

Step 5

Name the piece of apparatus used for these stages of the method.

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Answer

Taking the 25.0 cm³ portions: pipette.

Adding the potassium manganate(VII) solution: burette.

Step 6

Calculate the percentage uncertainty in using the balance.

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Answer

The mass measured is 680 mg with an uncertainty of ±0.005 g, which converts to ±5 mg.

Percentage uncertainty is calculated as follows:

ext{Percentage Uncertainty} = rac{ ext{Uncertainty}}{ ext{Measured Value}} imes 100 = rac{5}{680} imes 100 \\ ext{Percentage Uncertainty} ext{≈} 0.735 ext{\%}

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