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To find the relative atomic mass of a sample of Lithium.

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Introduction

Chemistry Coursework Glen Musgrove 01/04/03 Aim To find the relative atomic mass of a sample of Lithium Safety When carrying out this practical wear safety glasses at all times. Where the rubber gloves at all times and make sure there are no holes or tears. When filling the burette hold the funnel slightly above the rim so as to break the seal allowing air to leave and fluid to replace it instead of the funnel filling up. The above helps to prevent you over filling the burette and it spilling over. Always fill the burette after moving it lower down so that if it does spill over it does not pour onto you or into your eyes. Wash your hands thoroughly after handling any Acids or Bases. Do not rub your eyes or put your hands or fingers in your mouth at any point during the experiment. Do not sit at the desk as this may prevent you moving in the event of a spillage. Lithium is a highly reactive element and must be treated with caution, it will react simply with the water that is in the air. This is why it is stored in oil to prevent it reacting. ...read more.

Middle

In experiment 1 the method had some very major inaccuracies, like the loss of gas when the Li sample was dropped in because it would be very difficult to put the bung on before it hit the water. This was what I accounted for as the main inaccuracy in experiment 1. However the experiment was still more accurate than the titration. The method of this experiment could have been improved. My suggestion would be to place the Li in an empty flask and then inject through a second glass tube and in air tight conditions the 100cm3 of water. Like shown below. This would minimise the amount of gas lost in the addition of the Li sample. By using a more accurate method of measuring the collected gas a better result could have been achieved because the 250cm3 measuring cylinder was not very accurate. In experiment 2 there were also some sources of error. But I got three results within 0.02cm3 of each other which leads me to believe that there was a major source of error in the first part of the experiment most likely the preparation of the Li sample and it's weighing. I must also account for the impurity of the sample, the second experiment was a titration which is generally very accurate. ...read more.

Conclusion

The Oil and the reactivity of the Lithium made it difficult to find. One way this could be improved would be to Dissolve some Lithium in a known weight of water filter off the oil and reweigh the LiOH solution accounting for the lost Hydrogen. However this would be difficult to achieve when adding such small amounts of Lithium as 0.1 gram. The first experiment although it had more obvious sources where error could occur turned out to be more accurate than the titration, which was inaccurate due to a combination of the above problems. The second experiment method could also have been improved by dissolving the LiOH solution by adding another 25cm3 so that there was plenty of solution to perform 4 titrations. When I analysed the results I gained by using the Mr of Li from the periodic table I found that the either the true mass of the Li was 0.12 grams or that the volume of HCl that I neutralised the solution with should have been 36cm3 which are too very large errors. I feel that repeats of this experiment would be useful to see if it was just human error that caused the problem, or whether the wrong indicator could account for the excess of HCl added. I thought the result I gained was not satisfactory and the margin of error for both experiments was too high. ...read more.

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