Sea water contains about 20gdm-³ of chloride ions, Cl-(aq). The exact concentration of Cl-(aq) in sea water can be determined by titration with aqueous silver ions, Ag+(aq), using aqueous potassium chromate(VI), K2CrO4(aq), as an indicator. When aqueous silver nitrate, AgNO3(aq), is added to a sample of sea water, silver ions react with chloride ions to form a precipitate of silver chloride. Ag+(aq) + Cl-(aq) → AgCl(s) When all of the Cl-(aq) has reacted with Ag+(aq), the presence of unreacted Ag+(aq) is detected by chromate(VI) ions, CrO42-(aq). A red precipitate of Ag2CrO4(s) is seen. 2Ag+(aq) + CrO42-(aq) → Ag2CrO4(s) The amount of Ag+(aq) reacting with Cl-(aq) in the sample of sea water can be calculated in order to determine the concentration of Cl-(aq) in the sample of sea water. A student uses the following method. step 1 Use a weighing boat to weigh by difference approximately 10.6g of AgNO3(s) into a 100 cm³ glass beaker. step 2 Use the sample of AgNO3(s) in the glass beaker to prepare 250.0 cm³ of AgNO3(aq). step 3 Transfer this solution into a dark brown glass bottle. Label this solution X. step 4 Collect a sample of sea water and remove any solid material present. step 5 Transfer 10.00cm³ of the sea water into a conical flask. step 6 Add 1 cm³ of K2CrO4(aq) to the conical flask. step 7 Rinse a burette in preparation for the titration. step 8 Fill the burette with solution X. step 9 Slowly add solution X to the conical flask until the white precipitate turns red. This is the end-point.

1. (a)Describe how the student should carry out step 1. Include a table in your answer to show how this process is recorded.[2]
2. (b)Describe how the student should prepare 250.0cm³ of AgNO3(aq) in step 2, starting with the AgNO3(s) in the 100 cm³ beaker in step 1.[3]
3. (c)Suggest why solution X is kept in a dark brown glass bottle in step 3 rather than a colourless glass bottle.[1]
4. (d)Suggest how solid material should be removed from sea water in step 4.[1]
5. (e)(i)Identify the most appropriate piece of equipment that you would use to: transfer 10.00 cm³ of sea water from the dark brown bottle to a conical flask in step 5[1]
6. (e)(ii)add 1 cm³ of K2CrO4(aq) to the conical flask in step 6.[1]
7. (f)Chromate(VI) solutions are known to be carcinogenic. State what precaution should be taken when using K2CrO4(aq) in step 6 other than wearing safety goggles.[1]
8. (g)State what the burette should be rinsed with in step 7.[1]
9. (h)(i)Complete Table 1.1.[1]
10. (h)(ii)Calculate the mean titre to be used in the calculations. Show your working. mean titre = ...................................cm³[1]
11. (h)(iii)Use the mean titre from (h)(ii) to calculate the concentration of chloride ions in the sample of sea water. Assume the mass of solid silver nitrate used in step 2 was 10.62g. concentration = ...................................moldm-³[3]
12. (h)(iv)Calculate the percentage error in the titre in titration 2. Show your working. percentage error = ...................................%[1]
13. (i)Spectroscopic analysis of the sample of sea water accurately determined the concentration of Cl-(aq) to be lower than that determined by titration with Ag+(aq). Suggest why the student's method gave a higher value.[1]