A student used a precipitation titration to determine the value of x in the formula of a sample of hydrated barium chloride, BaCl2 ∙xH2O.
Procedure
Step 1 |
Prepare a solution by dissolving 1.57 g of BaCl2 ·xH2O in deionised water, making the solution up to the mark in a 250.0 cm3 volumetric flask and then mixing thoroughly. |
Step 2 |
Use a pipette to transfer 10.0 cm3 of the barium chloride solution into a conical flask. |
Step 3 | Fill a burette with 0.0324 mol dm–3 silver nitrate solution. |
Step 4 | Add three drops of potassium chromate(VI) solution to the conical flask and titrate the contents, while swirling, with the silver nitrate solution. The end-point is shown by the appearance of a permanent pale red precipitate. |
Step 5 | Repeat Steps 2 to 4 until concordant results are obtained. |
During the titration, two precipitation reactions occur.
Reaction 1 | Silver ions react with chloride ions forming silver chloride. |
Ag+ (aq) + Cl– (aq) → AgCl (s)
Reaction 2 | Once all chloride ions have reacted, silver ions react with chromate(VI) ions to form a red precipitate of silver chromate(VI). |
2Ag+ (aq) + Cr (aq) → Ag2CrO4 (s)
Include state symbols.
(1)
Give a possible reason why it is necessary to add sodium sulfate solution.
Justify your answer.
(1)
Suggest why the red precipitate of silver chromate(VI) only forms after all the chloride ions have reacted.
Some data obtained in the experiment are shown.
Titration number | 1 | 2 | 3 | 4 |
Burette reading (final) / cm3 | 16.15 | 32.05 | 48.30 | 47.40 |
Burette reading (initial) / cm3 | 0.00 | 16.15 | 32.50 | 31.55 |
Titre / cm3 | 16.15 |
Use information from the procedure and your mean titre from (c)(i).
You must show your working.
(5)
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