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A student does an experiment to determine the percentage of copper in an alloy - AQA - A-Level Chemistry - Question 6 - 2019 - Paper 1

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A student does an experiment to determine the percentage of copper in an alloy. The student - reacts 985 mg of the alloy with concentrated nitric acid to form a solu... show full transcript

Worked Solution & Example Answer:A student does an experiment to determine the percentage of copper in an alloy - AQA - A-Level Chemistry - Question 6 - 2019 - Paper 1

Step 1

Calculate the percentage of copper by mass in the alloy.

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Answer

To calculate the percentage of copper, we need to determine the amount of copper in the 25.0 cm³ portion of the solution.

  1. Calculate moles of sodium thiosulfate used:

    M=C⋅VM = C \cdot V where C=0.0800 mol dm−3C = 0.0800 \ mol \ dm^{-3} and V=90.0 cm3=0.0900 dm3V = 90.0 \ cm^{3} = 0.0900 \ dm^{3}.

    M=0.0800 mol dm−3×0.0900 dm3=7.20×10−3 molM = 0.0800 \ mol \ dm^{-3} \times 0.0900 \ dm^{3} = 7.20 \times 10^{-3} \ mol

  2. Calculate moles of iodine reacted: Since 2 moles of sodium thiosulfate react with 1 mole of iodine:

    extMolesofI2=7.20×10−32=3.60×10−3 mol ext{Moles of } I₂ = \frac{7.20 \times 10^{-3}}{2} = 3.60 \times 10^{-3} \ mol

  3. Calculate moles of copper: From the reaction, we know that 1 mole of iodine reacts with 2 moles of copper:

    extMolesofCu=2×extMolesofI2=2×3.60×10−3=7.20×10−3 mol ext{Moles of Cu} = 2 \times ext{Moles of } I₂ = 2 \times 3.60 \times 10^{-3} = 7.20 \times 10^{-3} \ mol

  4. Convert moles of copper to grams: The molar mass of copper (Cu) is approximately 63.55 g/mol:

    extMassofCu=extMolesofCu×extMolarMassofCu=7.20×10−3 mol×63.55 g/mol=0.457 g ext{Mass of Cu} = ext{Moles of Cu} \times ext{Molar Mass of Cu} = 7.20 \times 10^{-3} \ mol \times 63.55 \ g/mol = 0.457 \ g

  5. Calculate percentage of copper in the alloy:

    extPercentageofCu=(0.457 g0.985 g)×100≈46.4% ext{Percentage of Cu} = \left( \frac{0.457 \ g}{0.985 \ g} \right) \times 100 \approx 46.4 \%

    Thus, the percentage of copper by mass in the alloy is approximately 46.4%.

Step 2

State the role of iodine in the reaction with sodium thiosulfate.

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Answer

Iodine acts as an oxidizing agent in the reaction with sodium thiosulfate, which means it accepts electrons during the reaction.

Step 3

Give the full electron configuration of a copper(II) ion.

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Answer

The full electron configuration for a copper(II) ion (Cu²⁺) is:

[Ar]3d9[Ar] 3d^9,

indicating that it has lost two electrons from its outer shell.

Step 4

Explain why copper(I) iodide is white.

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Answer

Copper(I) iodide is white because it reflects visible light without absorbing specific wavelengths. Its electronic structure does not allow it to absorb light in the visible spectrum.

Step 5

Calculate the volume, in cm³, that 5.00 g of iodine vapour occupies at 185 °C and 100 kPa.

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Answer

To find the volume occupied by iodine vapour:

  1. Using the Ideal Gas Law:

    PV=nRTPV = nRT where:

    • P=100 kPa=100,000 PaP = 100 \ kPa = 100,000 \ Pa

    • R=8.31 JK−1mol−1R = 8.31 \ J K^{-1} mol^{-1}

    • T=185°C=185+273.15K=458.15 KT = 185 °C = 185 + 273.15 K = 458.15 \ K

    1. Calculate moles of iodine (I₂):
    • The molar mass of iodine (I₂) is approximately 253.81 g/mol:

    n=5.00 g253.81 g/mol≈0.0197 moln = \frac{5.00 \ g}{253.81 \ g/mol} \approx 0.0197 \ mol

    1. Calculate volume (V):

    Rearranging the Ideal Gas Law: V=nRTP=0.0197 mol×8.31 JK−1mol−1×458.15 K100000 Pa≈0.750 m3=750 cm3. V = \frac{nRT}{P} = \frac{0.0197 \ mol \times 8.31 \ J K^{-1} mol^{-1} \times 458.15 \ K}{100000 \ Pa} \approx 0.750 \ m³ = 750 \ cm³.

    Thus, the volume occupied by 5.00 g of iodine vapour at 185 °C and 100 kPa is approximately 750 cm³.

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