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Determination of Relative atomic mass of lithium

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Introduction

Assessed Practical 3 Determination of Relative atomic mass of lithium Jared Ching Evaluation: In general the accuracy of the experiments was good. However with practice, the accuracy could be improved; accuracy being how close the results are to the real answer. I think I have carried pit the experiment as well as I possibly could at the time with the given conditions and time period allowed. I have gained precise results from the practical. However, looking back at the analysis section, the relative atomic mass values I found were not exactly 6.9 as presented in the periodic timetable. Method (1) gave me 8.28; a difference of 1.38 and method (2) gave me 7.37, a difference of 0.43 to the real/reference value. Therefore method (2) proved to be more accurate. Overall the results were good and came close to the real values. There were no anomalous results. Limitations: The main sources of error that I found to be evident and which were out of my control were as follows: * In method (1), due to the oil that lithium is stored in, not only ...read more.

Middle

I also found that the reacted solution sometimes returned to a slight pink colour even after turning completely colourless. Thus this caused inevitable error. Accuracy and reliability: In method (1) the procedure was very accurate; the equipment used is very accurate, however as mentioned before, the gas lost when replacing the bung caused a degree of inaccuracy. As shown by the results, with the relative atomic mass found to be +1.38 over. In method (2) not only were the results accurate but also precise. The final relative atomic mass coming out as only +0.43 over the reference value proves a high degree of accuracy. Looking at the results table, the three accurate titrations fall within 0.1cm3 and 0.3cm3 of each other, which is a very concurrent set of results. Clearly method (1) and method (2) differ in accuracy and precision. Method (1) is clearly less accurate and precise as the gas collected from the reaction of lithium and water was not exact as hydrogen was lost in the process. ...read more.

Conclusion

Due to the fact that the colour change and when to stop the tap is not very obvious, more indicator should be used in order to make the more subtle changes in colour more visible. The burette tap should be opened at full until there is a small change in colour, then the tap should be closed slightly so that only drops of acid are coming through, this will improve accuracy as it will be clearer when the reaction is complete, thus more concurrent results will be produced. Clearly there is also a need for a control, although it was initially thought that it was very obvious when the solution turns colourless, this practical has shown that a control would be useful to compare the colours. However the other problem of the solution returning to a slight pink colour can be eliminated by waiting for this to occur, and when it does, titrate it further opening the burette very slightly, allowing acid to be added more slowly into the solution. Carrying out titrations is a skill that develops with practice, more repetitions will lead to improved results, both in accuracy and precision. Both methods are the most accurate, considering the equipment available. ...read more.

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