The concentration of aqueous chloride ions can be found by titration with aqueous silver nitrate, AgNO3(aq). Ag+(aq) + Cl-(aq) → AgCl(s) The indicator used is aqueous potassium chromate(VI), K2CrO4(aq). As AgNO3(aq) is added to aqueous chloride ions, a white precipitate of AgCl(s) is formed. When all the chloride ions have reacted, further addition of AgNO3(aq) leads to the formation of a red precipitate of silver chromate(VI), Ag2CrO4(s). The first appearance of the red precipitate shows the end-point of the titration. A student carries out an experiment to determine the number of molecules of water of crystallisation, x, in hydrated barium chloride, BaCl2•xH2O(s).

1. (a)(i)Calculate the mass of solid silver nitrate, AgNO3(s), needed to make 250.0 cm³ of 0.0500 mol dm⁻³ AgNO3(aq). Give your answer to two decimal places.[1]
2. (a)(ii)Describe how the student should make 250.0 cm³ of 0.0500 mol dm⁻³ AgNO3(aq) starting from the mass of AgNO3(s) calculated in (a)(i) in a 50 cm³ beaker. Give the name and size of any key apparatus used. Write your answer using a series of numbered steps.[3]
3. (b)The student uses the following method. step 1 Dissolve 1.58g of BaCl2•xH2O(s) to form 250 cm³ of aqueous solution. Label this solution A. step 2 Transfer 20.0 cm³ of solution A into a conical flask. step 3 Add aqueous sodium sulfate, Na2SO4(aq), to the flask and swirl the mixture to remove barium ions from the solution. step 4 Add 2-3 drops of K2CrO4(aq) indicator to the flask. step 5 Titrate the contents of the flask against 0.0500 mol dm⁻³ AgNO3(aq). step 6 Repeat steps 2 to 5 to collect sufficient data for analysis.
4. (b)(i)Suggest a suitable piece of apparatus for transferring 20.0 cm³ of solution A in step 2.[1]
5. (b)(ii)Suggest why barium ions are removed in step 3 before performing the titration.[1]
6. (b)(iii)Suggest why chemically resistant gloves should be worn to carry out step 4.[1]
7. (c)The student's results are shown in Table 1.1. Table 1.1 | | rough titration | titration 1 | titration 2 | titration 3 | | :-------------------- | :------------- | :----------- | :----------- | :----------- | | burette reading (final)/cm³ | 20.10 | 40.55 | 20.75 | 20.90 | | burette reading (initial)/cm³ | 0.00 | 20.25 | 0.05 | 0.30 | | titre/cm³ | 20.10 | 20.30 | 20.70 | 20.60 | The student uses the titres from titrations 2 and 3 shown in Table 1.1 to calculate a mean titre value of 20.65 cm³.
8. (c)(i)Explain why only these two values are used.[1]
9. (c)(ii)Calculate the percentage error in the titre volume for titration 3. Show your working.[1]
10. (d)The equation for the reaction of silver nitrate with barium chloride is shown. 2AgNO3(aq) + BaCl2(aq) → Ba(NO3)2(aq) + 2AgCl(s)
11. (d)(i)Calculate the amount, in mol, of AgNO3(aq) in the mean titre of 20.65 cm³.[1]
12. (d)(ii)Calculate the amount, in mol, of BaCl2(aq) in 250 cm³ of solution A.[1]
13. (d)(iii)Calculate the value of x in the formula BaCl2•xH2O.[2]
14. (e)Another student uses a different experimental method to check the value of x obtained by the method described in (b). Give a brief description of another method, not involving titration, that could be used to determine the value of x in the formula BaCl2•xH2O(s). Write your answer using a series of numbered steps. Your plan should include details of the following: • the apparatus and method you would use • the measurements you would make. You are provided with standard laboratory apparatus.[3]