Ar = mass/moles = 0.13/0.016 = 8.125
The actual atomic mass of lithium on the periodic table is 6.9.
The higher result of the atomic mass maybe due to some of the hydrogen gas escaping, this is a systematic error.
Method 2 – Procedure
- Pipette 25.0cm³ of solution in the conical flask from method 1 into a clean 250cm³ conical flask and add 5 drops of phenolphthalein indicator.
- Titrate with 0.100 mol dm³ HCl(aq).
- Record your results in an appropriate format.
- Repeat the titration to obtain consistent results. Show all of your results.
- Record average titre.
Results –
LiOH (aq) + HCl (aq) LiCl (aq) + H O (l)
²
Average result of titration is 42.3 ml. Titre number 4 was not included as it was an anomalous result.
Number of moles of HCl used in the titration –
Moles = Volume x Concentration
= Volume/1000 x Concentration = Moles
= 42.3/1000 x 0.100 = 0.00423
= 4.23 x 10³
Deducing the number of moles of LiOH used in the titration –
LiOH (aq) + HCl (aq) LiCl (aq) + H O (l)
²
From the equation it can clearly be seen that the ratio of LiOH to HCl is 1:1.
Therefore the number of moles of LiOH is the same as the number of moles of HCl, which is 0.00423M.
Calculating the number of moles of LiOH present in 100cm³ of the solution from Method 1 –
Moles of LiOH = 0.00423 x 4
= 0.01692M
= 1.692 x 10ֿ²
Calculating the relative atomic mass of lithium from the titration –
Ar = Mass of lithium/ Mass of lithium hydroxide
=0.130g/0.01692
=5.9101655
Atomic mass of lithium = 5.910
Evaluation -
Overall, method 2, the titration is more accurate than the measuring of the hydrogen gas produced in method 1.
The result I obtained in method 1 was 8.125 and in method 2 it was 5.910. Therefore the result I obtained from method 2, the titration was far more accurate than the result of method 1.
Method 1 -
The main source of error in method 1 was the collection of hydrogen gas. Whilst putting the bung on there was a loss of hydrogen gas. The time delay of putting the bung on squeezes Hydrogen out of the tube, therefore there is an increase of Hydrogen.
There are imprecision’s to the experiment as well. The balance may have had an imprecision of +/- 5% that may have affected the result as the mass of lithium may have been incorrectly weighed. The mass of lithium I used was 0.130g and so 5% either way means that the actual mass of the lithium could have been 0.1349 (5% increase) or 0.1295% (5% decrease). There is an imprecision of the 250ml-measuring cylinder as I may have over or under estimated the value. The percentage error of the measuring cylinder is +/- 1ml that is a1.5-2% error. Over estimating the mass of lithium used in the experiment is also an error. Not all of the mass may have actually been lithium. This is because not all of the oil may have been removed when dried; the outside of the lithium was also exposed to oxygen. Therefore it was not actually lithium but lithium oxide.
Method 2 –
Again the mass of the lithium is an error as part of the mass may have been lithium oxide or oil. There is an imprecision of the experiment that is the pipette measurement, although this is insignificant and would not have affected the result. Deciding when the indicator is not present may have affected the result because it may have been to early of late. This is human error as it is our own decision.
The titration is not a significant source of imprecision. I used 3 significant figures for the concentration of the acid, which was very accurate.
Therefore the titration was more accurate as there were fewer inaccuracies. In method 1 the mass of the lithium and the loss of hydrogen gas may have affected the results greatly. The titration is more reliable as there are no errors apart from the lithium coating and this occurred in both if the experiments.
If I were to repeat the experiment I would make modifications to it to eliminate the main sources of inaccuracy that is the weighing and the collection of the hydrogen gas. Therefore using a syringe will eliminate the inaccuracy of the collection of the hydrogen gas. There is another problem of the hydrogen gas, which is that the actual gas might not be 100% hydrogen. But, that doesn’t matter because all gases take up 24 dm³ (1 mole) at room temperature and pressure. There is not a great deal of difference with this factor as it is not significant. The inaccuracy of the weighing scales cannot be eliminated, as there will always be a slight degree of inaccuracy but weighing the mass of lithium to a larger amount of significant figures can reduce this. Also, two different weighing scales could be used to make sure the same amount of lithium gave the same mass when weighed on two different scales.
