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

To calculate the mean titre:

1. Add the titre values:

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

2. Mean titre calculation: ext{Mean Titre} = rac{22.90 + 22.70 + 22.60}{3} = rac{68.20}{3} = 22.73 ext{ cm³}

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

The colour change seen at the end point of the titration is from colourless to pale pink.

The apparatus used for these stages of the method includes:

• For taking the 25.0 cm³ portions: Pipette.
• For adding the potassium manganate(VII) solution: Burette.

To calculate the percentage uncertainty:

1. The mass of the iron wire is 680 mg, which is equal to 0.680 g.
2. The absolute uncertainty is ±0.005 g.
3. Percentage uncertainty can be calculated using: ext{Percentage Uncertainty} = rac{ ext{Absolute Uncertainty}}{ ext{Measured Value}} imes 100

Thus: ext{Percentage Uncertainty} = rac{0.005}{0.680} imes 100 \\ = 0.735 ext{%}