Moles = Concentration * Volume
= 0.1 * 27.1
= 2.71moles in ratio 1:1 = 2.71moles
Mass = Moles * RFM
= 2.71 * (6.9+16+1)
= 64.77 g
Concentration = Moles
Vol.
= 2.71
25
= 0.1084
Conc. = 0.1084 molar
Mass = 64.77g
Moles = 2.71 mol
Moles of Li in 100cm3 = 0.1084 * 100 = 10.84 moles
Mass of LiOH = 10.84 * 24 = 260.16 g
Mass of Li = (260.16/24) * 6.9 = 74.796 g
Calculated RAM of Li = 10.835 g
Analysis
Both experiments showed that the relative atomic mass of Lithium is around 11g. If you look at the periodic table then you can see that the actual relative atomic mass of lithium is 6.9g, so my results are about 4g out.
The first experiment was the collection of H2 by adding Li to distilled water, so that it reacts to form Hydrogen and Lithium Hydroxide. Using the volume of hydrogen I was able to calculate the moles of hydrogen. I used this work out the moles of Lithium from which I could work out the mass and RAM of Lithium.
The second experiment was the titration of the lithium hydroxide that was a by-product of the first experiment. By measuring the amount of hydrochloric acid (of a known molarity) I was able to work out the RAM of Lithium.
The average of the two RAM’s that I found is:
10.835 + 11.16 = 10.99
2
Evaluation
Both the results for the RAM that I recorded are around 11g, so any errors that have occurred have obviously affected all the results. Though the second result could be affected by more errors because it has the errors of the first results, and the risk of more errors in the second experiments.
Errors could have occurred in the weighing of the Lithium, as there is still a reasonable amount of oil on the Lithium, and if you had got all the oil off then it would have oxidized before you could get it onto the water.
Another source of error in the measuring of the amount of Hydrogen given off is that you have to drop the lithium onto the water, then put the bung on the flask. So some gas could escape before the bung was on. The accuracy of the mass of Lithium is negligible because they are electronic devices. The accuracy of the measurement of 100cm3 of distilled water is about +- 1cm3 because I measured it using a measuring flash. The error in the amount of LiOH transferred from the 1st experiment to the second is negligible because I used a volumetric pipette, which is setup to contain an exact amount. The error in the amount of HCl is about 0.1cm3 because the graduations on the pipette are 0.1cm3 apart.
Taking into account all these errors the results that I obtained aren’t very accurate, as the results show as they are about 4g off the proper RAM of Lithium.
These results could be improved by using a 100cm3 volumetric pipette to measure the distilled water. This would make the error almost negligible. You could devise a system where the Lithium could be added when the bung was already in place, therefore there would be no gas escape. You could eliminate the error in HCl by using burette with smaller graduations.
The overall percentage error is:
10.99 * 100 = 159 %
6.9
This experiment has basically gone smoothly, there were no major hiccups along the way, and although my results aren’t entirely accurate, at least they are consistent. I conclude that the reason for this is that the apparatus that I used was not appropriately accurate for the experiment.
