1 mole of gas occupies 24000cm³ at RTP
Volume of H2 gas evolved = 188cm³
188 / 24000 = 0.0078 moles
0.0078 2 = 0.016 moles (2sf)
Ar = 0.10 / 0.016 = 6.383 (3dp)
Calculations – method 2
LiOH + HCl LiCl + H2O
1 mole 1 mole
0.10 34.96 / 1000 = 0.003496 moles
= 0.0035 (2sf) moles in 25cm³of LiOH
0.003496 x 4 = 0.013984 moles
= 0.014 (2sf) moles in 100cm³ of LiOH
Ar = 0.10 / 0.013984
=7.15 (2dp)
Evaluation
Overall both of the procedures for finding the relative atomic mass of lithium were relatively successful although none of them were 100% accurate:
- The biggest percentage error was in the weighing of the lithium because the scale only weighs to 2 decimal places therefore the mass could have been as much as 0.10g plus or minus 0.005g which gives a 5 percent error margin. The final result of the atomic mass is then affected by 5% which could have been 6.383 plus or minus 0.31915.
- Another factor that could have affected the results is the time delay between putting the lithium in the solution and putting the bung on. Hydrogen could have escaped during this delay causing approximately a 1% error in the results.
Therefore, altogether, procedure one could have a maximum of a 6% error within the results.
Procedure 2 however is slightly more accurate because using a burette is much more accurate with only a possible 0.1cm error which is an approximate 0.28% error. This gives the procedure a possible total of 5.28% plus or minus of the final result.
Also, lithium is stored in oil to prevent it reacting with the air so it is possible that when weighing the lithium, a small portion of this could have been oil included into the mass. To overcome this experimental error, the oil could be washed off with organic tetrochloromethane although the lithium would then be exposed to the air and react forming a lithium oxide. Therefore, to accurately weigh the lithium with no oil it would have to be removed and then weighed in a vacuum like container.
To overcome the delay in the time the lithium is put in the solution and the time the bung is put on the conical flask, a shelf like product could be attached already inside the conical flask so that the lithium could be placed on this and then simply shook off when you are ready to begin the experiment. This will prevent any hydrogen from escaping in the time it takes to put the bung on.
To overcome the inaccuracy of weighing the lithium (apart from using a more accurate scales) a larger mass could be used for the experiment as this would produce a smaller percentage error because e.g. 5 % of 1.00g is far less than 5% of 0.1g.
If all of these factors were changed, it would improve the overall accuracy of the experiment and the reliability of the results.
