A student uses the following method to determine the percentage by mass of the painkiller aspirin, C9H8O4(s), in some tablets. step 1 Grind five tablets into a powder. step 2 Use a weighing boat to accurately weigh by difference approximately 0.4 g of powdered tablets into a pear-shaped flask containing anti-bumping granules. step 3 Add 25 cm³ of aqueous 1 moldm¯³ sodium hydroxide, NaOH(aq), to the pear-shaped flask, forming mixture A. step 4 Reflux mixture A for 20 minutes. step 5 Allow mixture A to cool and then filter into a small beaker. Label the filtrate solution B. step 6 Add 30cm³ of alkaline aqueous iodine to solution B and leave to stand for 1 hour. A precipitate, C, (C6H2I2O)2(s), will form. step 7 Filter the resulting mixture under reduced pressure. Wash the residue, C, with a small volume of cold distilled water. step 8 Allow solid C to dry. step 9 Weigh solid C and record its mass. Alkaline aqueous iodine is irritating to the skin and eyes.

1. (a)Identify an appropriate precaution, other than eye protection and a lab coat, that the student should take when using alkaline aqueous iodine.[1]
2. (b)Describe how the student should carry out step 2. Include a results table, with appropriate headings, for the student to fill in.[2]
3. (c)Complete Fig. 1.1 to show how step 4 is carried out in the laboratory. Label your diagram fully. [Figure 1.1][2]
4. (d)(i)The student uses a measuring cylinder to measure the volume of alkaline aqueous iodine in step 6. Suggest why this is a suitable piece of apparatus to use.[1]
5. (d)(ii)Suggest why the student leaves the mixture to stand for 1 hour in step 6.[1]
6. (d)(iii)Explain why the residue is washed in step 7.[1]
7. (d)(iv)Explain why hot distilled water is not used in step 7.[1]
8. (e)(i)Calculate the amount, in mol, of (C6H2I2O)2(s) collected in step 9. [Mr: (C6H2I2O)2, 687.6] amount of (C6H2I2O)2 .................................... mol[1]
9. (e)(ii)Use your answer to (i) to calculate the mass, in g, of C9H8O4(s) in the powdered tablets added to the flask in step 2. mass of C9H8O4(s) ....................................g[1]
10. (e)(iii)Use your answer to (ii) to calculate the percentage by mass of aspirin, C9H8O4(s), in the tablets. If you were unable to obtain an answer to (ii) you may use 0.374g for the mass of C9H8O4(s). This is not the correct value. percentage by mass C9H8O4(s) in the tablets ..................................[1]
11. (f)Another student follows the same method but does not allow solid C to dry completely in step 8. State and explain the effect that this has on the calculated percentage by mass of aspirin, C9H8O4(s), in the tablets.[1]

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