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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 equation for the reaction between iron and sulfuric acid is:

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 add the titres and divide by the number of titrations:

Mean titre = ( rac{22.90 + 22.70 + 22.60}{3} = 22.73 , \text{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 is:

extMnO4−+5extFe2++8extH+→extMn2++5extFe3++4extH2extO ext{MnO}_4^- + 5 ext{Fe}^{2+} + 8 ext{H}^+ \rightarrow ext{Mn}^{2+} + 5 ext{Fe}^{3+} + 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 to pale pink.

Step 5

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

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Answer

For taking the 25.0 cm³ portions, a pipette is used.

For adding the potassium manganate(VII) solution, a burette is used.

Step 6

Calculate the percentage uncertainty in using the balance.

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Answer

The percentage uncertainty can be calculated using the formula:

extPercentageUncertainty=(extUncertaintyextMeasurement)×100overtheuncertaintyinthemassofwire(0.005g)andmeasuredmass(0.680g): Percentage Uncertainty=(0.0050.680)×100=0.735 or≈0.74%. ext{Percentage Uncertainty} = \left( \frac{ ext{Uncertainty}}{ ext{Measurement}} \right) \times 100 over the uncertainty in the mass of wire (0.005 g) and measured mass (0.680 g): \ \text{Percentage Uncertainty} = \left( \frac{0.005}{0.680} \right) \times 100 = 0.735\, \text{or} \approx 0.74\%.

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