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Titrations using ethylenediaminetetraacetic acid (EDTA) can be used to determine the concentration of metal ions in solution, such as Zn²⁺(aq) - CIE - A-Level Chemistry - Question 1 - 2016 - Paper 1
Question 1
Titrations using ethylenediaminetetraacetic acid (EDTA) can be used to determine the concentration of metal ions in solution, such as Zn²⁺(aq). A solution of EDTA i... show full transcript
Worked Solution & Example Answer:Titrations using ethylenediaminetetraacetic acid (EDTA) can be used to determine the concentration of metal ions in solution, such as Zn²⁺(aq) - CIE - A-Level Chemistry - Question 1 - 2016 - Paper 1
Step 1
Explain why the pH would change during the titration if the buffer were not present.
Answer
If a buffer is not present during the titration, the addition of EDTA would lead to a change in the concentration of H⁺ ions in the solution. As Zn²⁺ ions react with EDTA to form the complex ion ZnY²⁻, the reaction consumes H⁺ ions, which could lead to an increase in pH. Without a buffer to stabilize the pH, the solution could fluctuate as the reaction proceeds, leading to inaccurate titration results.
Step 2
Calculate the mass of hydrated disodium EDTA that would be required for the preparation of a standard solution of concentration 0.100 mol dm⁻³, using the apparatus you have specified in (i).
Answer
To prepare a 0.100 mol dm⁻³ solution of hydrated disodium EDTA, we first need to determine the volume of the solution being prepared. For a 250 cm³ volumetric flask, we convert this volume to dm³:
Next, calculate the number of moles needed:
Number of moles = concentration × volume = 0.100 ext{ mol dm}^{-3} × 0.250 ext{ dm}^3 = 0.0250 ext{ mol}
Now, to find the mass of hydrated disodium EDTA, we use its molar mass. The molar mass of Na₂Y·2H₂O is approximately 372.2 g/mol, so:
Mass = number of moles × molar mass = 0.0250 ext{ mol} × 372.2 ext{ g/mol} ≈ 9.30 ext{ g}
Thus, 9.30 g of hydrated disodium EDTA is required.
Step 3
Describe how you would prepare this standard solution for use in your titration.
Answer
To prepare the standard solution, follow these steps:
- Measure out 9.30 g of hydrated disodium EDTA accurately using a balance.
- Dissolve this solid in a small amount of distilled water in a beaker. Stir with a glass rod until completely dissolved.
- Transfer the solution to a 250 cm³ volumetric flask. Use a funnel for ease of transferring.
- Rinse the beaker with distilled water to ensure all EDTA is transferred to the volumetric flask.
- Add distilled water to the volumetric flask until the bottom of the meniscus is at the 250 cm³ mark, ensuring the solution is at the desired volume.
- Invert the flask several times to mix the solution thoroughly.
Step 4
After you have performed a rough titration, how would you ensure that your next titration is accurate?
Answer
To ensure accuracy in subsequent titrations, I would carry out the following steps:
- Rinse all volumetric glassware (burette, pipette, and conical flask) with the solutions they will contain to avoid contamination.
- Use a clean pipette and burette to measure the volumes accurately.
- Perform the titration slowly and meticulously, adding EDTA dropwise as the endpoint approaches.
- Repeating the titrations at least three times allows for an average to be calculated, providing a more reliable result.
Step 5
How would you ensure that your titration result is reliable?
Answer
To ensure reliability of my titration results, I would:
- Conduct multiple titrations (at least three) and average the results to account for any discrepancies.
- Ensure consistent technique in adding the titrant and timing the endpoint.
- Calibrate the measuring devices to ensure accuracy.
- Use the same concentration of EDTA and maintain the same conditions (temperature, etc.) during each titration.