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Edexcel IAL Chemistry AS 3.P17 Compound preparation methods

Edexcel IAL Chemistry AS 3.P17 Compound preparation methods
Pearson Edexcel IAL Chemistry syllabusChemistry YCH11First assessment 2019

Choose filtration for an insoluble excess, evaporate to the crystallisation point before cooling and drying, and link support materials to retention or controlled vapour flow.

How this is tested

  • Use filtration to remove an insoluble excess and retain the required solution.
  • Obtain large dry crystals by partial evaporation, cooling, decanting and drying.
  • Explain mineral-wool use through liquid retention and controlled passage of vapour.

Question 3(c)(i)

[Maximum number: 2]

A student used a precipitation titration to determine the value of x in the formula of a sample of hydrated barium chloride, BaCl2⋅×H2O\mathrm{BaCl}_{2} \cdot \times \mathrm{H}_{2} \mathrm{O}.

Procedure

Step 1 Prepare a solution by dissolving 1.57 g of BaCl2⋅×H2O\mathrm{BaCl}_{2} \cdot \times \mathrm{H}_{2} \mathrm{O} in deionised water, making the solution up to the mark in a 250.0 cm3250.0 \mathrm{~cm}^{3} volumetric flask and then mixing thoroughly.

Step 2 Use a pipette to transfer 10.0 cm310.0 \mathrm{~cm}^{3} of the barium chloride solution into a conical flask.
Add excess sodium sulfate solution and swirl the mixture.
Step 3 Fill a burette with 0.0324 moldm−30.0324 \mathrm{~mol} \mathrm{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)\mathrm{Ag}^{+}(\mathrm{aq})+\mathrm{Cl}^{-}(\mathrm{aq}) \rightarrow \mathrm{AgCl}(\mathrm{~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)+CrO42−(aq)→Ag2CrO4( s)2 \mathrm{Ag}^{+}(\mathrm{aq})+\mathrm{CrO}_{4}^{2-}(\mathrm{aq}) \rightarrow \mathrm{Ag}_{2} \mathrm{CrO}_{4}(\mathrm{~s})

Some data obtained in the experiment are shown.

Table for Question 3(c)(i) — Edexcel A-Level Chemistry AS

Complete the table and use the concordant results to calculate the mean titre.
(2)