This question is about compounds containing ethanedioate ions - AQA - A-Level Chemistry - Question 11 - 2017 - Paper 1

From the equation, the mole ratio of MnO4⁻ to C2O4²⁻ is 1:5. Therefore, the moles of C2O4²⁻ reacted can be calculated as:

In the second titration, we find the amount of H2C2O4 used. The volume of NaOH used is 10.45 cm³ = 0.01045 dm³, and the concentration is 0.100 mol dm⁻³:

From the equation, 1 mole of H2C2O4 reacts with 2 moles of OH⁻:

Now, the total moles of oxalate in the original sample is:

To find the mass of sodium ethanedioate (Na2C2O4) in the sample, we use its molar mass:

• Molar mass of Na2C2O4 = 2(23) + 2(12) + 4(16) = 98 g/mol.

Now, we calculate:

Finally, to calculate the percentage of sodium ethanedioate in the original sample:

This indicates that there are significant quantities of sodium ethanedioate, suggesting that not all of the mass may be contributing to the sample tested.

The ligand substitution reaction between ethanedioate ions and iron(III) ions can be represented as:

The value of the enthalpy change for this ligand substitution reaction is close to zero because the reaction involves the substitution of one ligand for another without altering the overall structure, resulting in minimal change in energy.

The displayed formula of the iron complex [Fe(C2O4)3]³⁻ can be illustrated with the iron atom bonded to three ethanedioate ligands:

     O       O
      \     /
       C -- C
        |   |
      O     O
        \   /
         Fe
        /   \
       O     O
      /       \
     C         C
      \       /
       O     O

The iron is at the center with the ethanedioate ligands surrounding it.

The bond angle O—Fe—O in the iron complex is typically around 90° or 180°, depending on the arrangement of the ligands.

The type of isomerism shown by the iron complex is geometrical isomerism, due to the possible different spatial arrangements of the ligands.