Edexcel IAL Chemistry 3.P17 Compound preparation methods

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.

[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, $\mathrm{BaCl}_{2} \cdot \times \mathrm{H}_{2} \mathrm{O}$ .

Procedure

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

Step 2 Use a pipette to transfer $10.0 \mathrm{~cm}^{3}$ of the barium chloride solution into a conical flask.
Add excess sodium sulfate solution and swirl the mixture.
Step 3 Fill a burette with $0.0324 \mathrm{~mol} \mathrm{dm}^{-3}$ 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.

\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).

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.P17—Measurements and data in Units 1 and 2 question 1 — Edexcel A-Level Chemistry AS

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

An answer that makes reference to the following points:

- three values correctly recorded in table

(1)

- calculation of mean titre to 2DP from concordant results (1)

Example of completed table and calculation:

Figure for Answer to Question 3.P17—Measurements and data in Units 1 and 2 question 1 — Edexcel A-Level Chemistry AS

mean titre $=\frac{(15.9(0)+15.8(0)+15.85)}{3}$ $=15.85\left(\mathrm{~cm}^{3}\right)$

TE on averaging of concordant results from incorrect subtraction in table

Do not award 15.85 from $(15.90+15.80) \div 2$

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3.P17—Measurements and data in Units 1 and 2 question 1