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Practical 3

Practical 3. Determination of the dissociation constant of a weak acid. A Making up indicator solutions. 1. Label a test tube A. 2. Into this labelled test tube add bromocresol green solution (5cm 3 ) and 1 drop of conc. HCl. 3. Label a test tube B.

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Practical 3

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  1. Practical 3 Determination of the dissociation constant of a weak acid

  2. A Making up indicator solutions

  3. 1. Label a test tube A

  4. 2. Into this labelled test tube add bromocresol green solution (5cm3) and 1 drop of conc. HCl

  5. 3. Label a test tube B

  6. 4. Into this labelled test tube add bromocresol green solution (5cm3) and 1 drop of sodium hydroxide (4M)

  7. 5. Arrange 18 test tubes in two parallel rows on nine tubes6. Number the tubes as in the following table Note: it must be possible to see to look through each of the 9 pairs of tubes to see the combined colour from the solution in both tubes

  8. 7. Use a burette to deliver 10cm3 of water into each tube

  9. 8. Add the required drop of A or B

  10. and mix thoroughly

  11. B. Measuring the Ka for ethanoic acid using the indicator method

  12. 1. Mix ethanoic acid (0.02M, 5cm3) and sodium ethanoate (0.02M, 5cm3)

  13. 2. Add bromocresol green (10 drops) and mix thoroughly

  14. 3. Compare the colour of this mixture with colours seen through corresponding pairs of test tubes in the rack (ie tubes 1 and 10, 2 and 11, 3 and 12, etc)

  15. C. Determining the Ka for ethanoic acid using a pH meter

  16. 1. Place ethanoic acid (0.02 M, 10cm3) and sodium ethanoate (0.02 M, 10cm3) in a small beaker

  17. 2. Mix thoroughly

  18. 3. Measure the pH of the solution using the pH meters provided to each bench Seek assistance from you bench tutor on how to use the pH meters

  19. Chemical Disposal and Cleanup • All excess chemicals can be put down the sink with running water. • All unused chemicals are to be returned to the trolley from which they came. • The pink tray must be returned to the trolley complete with the original contents. • The beakers and test tubes are to go in the white washing up bins located in the centre of the lab. • When all students have completed their work the burettes are to be returned to the grey square plastic box located at the end of the bench where they were retrieved.

  20. Practical 3 adapted from: Hill G & Holman J, Chemistry in Context Laboratory Manual and Study Guide, 2nd ed.,Thomas Nelson and Sons Ltd, 1989

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