Calculations for water displacement:
2Li (s) + 2H2O (aq) → 2 LiOH(aq) + H2 (g)
Titration
Conclusion:
The aim of our experiment was to find out the molar mass of the lithium sample and compare the molar mass to the actual molar mass of lithium from the periodic table. The periodic table states the molar mass of lithium is 6.941. From the water displacement method the molar masses that we obtained from the two separate tests were 9.8 ± 0.7 and 10.7 ± 0.6 . These values were quite different from the actual value for the molar mass of lithium with percentage deviations of 41% and 54% respectively. Using the titration method the results obtained using different masses of lithium were both 11.1. I believe this is chance more than anything because the value of 11.1 had a percentage deviation from the actual value of around 60%. Our random error of ±0.5 also does not cover the deviation which means there is a lot of systematic error in our experiment.
Evaluation:
We did not have enough of the Lithium Hydroxide solution to do any more than 3 trials. Although our three trials for each of the titrations were within 0.2 ml it would have been more accurate if we had more trials.
To get a more accurate result it is better to use more lithium because increasing the amount of lithium decreases the percentage uncertainty. This is proved in our water displacement experiment. When using 0.035 the percentage uncertainty was 2.86% whereas the 0.040 sample had a percentage uncertainty of 2.50%. This proves that it is better to use a bigger sample to cut down random error.
A MAIN problem with the water displacement experiment is that in between putting the Lithium into the conical flask and putting on the bung on the conical flask the is a time when hydrogen has escapes means that less water would be displaced. Even 10ml of gas escaped and not displaced would dramatically change our results. More displacement means more moles which in turn means a lower molar mass. The lower molar mass would be closer to the accurate value which shows that escaping gas was a major problem.
Another problem with the experiment is that some of the Lithium had already oxidised. This Lithium oxide will not react with the water meaning that the actual mass of Lithium that reacts is smaller than the mass of Lithium that was weighed. A small mass would mean that the molar mass would be less. Because of the equation Mr = m / N . A smaller molar mass would mean that it would also be closer to the actual value of 6.941.
Improvements:
To solve the escaping gas problem an idea would be to suspend the Lithium on a piece of paper over the conical flask. You would then shove the bung into the paper with the lithium and into the conical flask. This would mean there would be no time for any gas to escape meaning the results would be more accurate.
Another improvement is we could have used absolute pure non oxidised lithium because the lithium oxide or oxidized Lithium does not react with the water.