Determination of the Relative Atomic Mass of Lithium

Experiment 2

1. I will arrange the apparatus as shown in fig. 1b
2. First, I will fill the burette with the standardised hydrochloric acid.
3. Then I will pipette 25cm3 of the solution from experiment 1 into a clean conical flask.
4. I will add 5 drops of phenolphthalein indicator to the solution in the flask.
5. I will titrate the solution with the hydrochloric acid and record the results.
6. I will repeat the titration twice more.

Analysis

Experiment 1

I used 100cm3 of H2O which reacted with 0.09g of Lithium to produce 158 cm3 H2 gas.

Since 1 mole of gas at room temperature and pressure occupies 24 dm3:

Moles of H2 = 158 x 10-3    =    6.6 x 10-3 mol

      24

Since 2 moles of Lithium produces 1 mole of Hydrogen:

  2 (6.6 x 10-3) mol of Lithium = 0.09g

0.0132 mol of Lithium = 0.09g

         1 mol of Lithium = 0.09g

                                   0.132g

                             = 6.8g

Thus the Relative Atomic Mass of Lithium is 6.8g mol-1 from experiment 1.

Experiment 2 

The results for the titrations are as follows;

28.3 cm3, 27.0 cm3 and 27.0 cm3. I will not take into account the first result because it is anomalous. Thus the average of the other two results is 27.0 cm3

It takes 27.0 cm3 of HCl to neutralise 25.0 cm3 of LiOH

Amount of acid = Concentration of acid x Volume of acid

Volume of acid = 27.0 x 10-3 dm3

Concentration of acid = 0.103 mol dm-3

Amount of acid = (27.0 x 10-3) x 0.103 = 2.8 x 10-3 mol

mol of HCl = mol of LiOH

Volume of base = 25.0 x 10-3 dm3

Amount of base = 2.8 x 10-3 mol

Concentration of base = 2.8 x 10-3      =    0.112 mol dm-3

                                   25.0 x 10-3

In 1 dm3 of base there is 0.0112 mol of Lithium Hydroxide

2 moles of Lithium produce 2 moles of Lithium Hydroxide thus stoichiometry = 1:1

0.0112 mol of Lithium hydroxide = 0.0112 mol of Lithium

0.0112 mol of Lithium = 0.09g

1 mol of Li = 0.09   = 8.0g

                0.0112

Thus the Relative Atomic Mass of Lithium is 8.0g mol-1 from experiment 2.

References for the Relative Atomic mass:

From Britannica Encyclopaedia 2001:

Natural lithium exists as two isotopes: lithium-7 (92.5 percent) and lithium-6 (7.5 percent) so the relative atomic mass of lithium is;

          (92.5 x 7) + (7.5 x 6)   =   6.925g mol-1

                    100

From The periodic table in Cambridge Chemistry 1:

The relative atomic mass of lithium is 6.9g mol-1

Results that form the graphs below are Data for Coursework with calculated values for Ar of Li from all students.

These graphs below represent the dispersal of the calculated values for the relative atomic mass of lithium from experiment 1 and 2.

By looking at the graphs above, it is evident that the majority of values for the Ar of lithium are between 6 and 8

For an average of all the results, I will add all the values together then divide the product by the number of values.

The average of the results including the anomalous ones in experiment 1 gives the value for the Ar of lithium as 7.22 and the standard deviation which describes the spread of the data is 1.03. This shows that the spread of the data is low and that the data is precise.

The average of the results including the anomalous ones in experiment 2 gives the value for the Ar of lithium as 7.79 and the standard deviation is 1.21. This shows that the spread of the data is low but higher than that of experiment 1.

There are a few anomalous results that are either too high or too low from the actual Ar of lithium.

I will exclude results that are more or less than 1 standard deviation from the mean in experiment 1 i.e. any results above 8.23 or below 6.17. The new mean of the data excluding the anomalous results is 7.11. This is more accurate than the previous mean. The new standard deviation is 0.19. This shows that the data is spread much less once the anomalous results have been omitted.

I will exclude results that are more or less than 1 standard deviation from the mean in experiment 1 i.e. any results above 9.00 or below 6.58. The new mean of the data excluding the anomalous results is 7.54. This is more accurate than the previous mean. The new standard deviation is 0.6. This shows that the data is spread much less with the exclusion of the anomalous results. However, this standard deviation is still higher than the one for experiment 1.

The above observations imply that the first experiment was more accurate than the second one. As the results are averaged, without the anomalous ones, they move closer towards the real value of the Ar of lithium.

Evaluation

On the whole, both experiments went exceptionally well. Most of my results were as I expected and I observed extreme safety throughout the experiments because we were using a lot of glass apparatus and corrosive acid for experiment 2. I always wore safety glasses and a lab coat. Some measurements were limited in their accuracy due to the apparatus used like the measurement of the collected hydrogen in the measuring cylinder which was measured to the nearest 2cm3.

But however careful and accurate one is, there are always factors that would affect the accuracy of the results from the experiments. These are the factors that might make the results I got slightly inaccurate:

Experiment 1

• The lithium was stored in petrolatum. When I weighed a piece of lithium, the weight I measured was that of the lithium and some petrolatum, although I wiped off as much of the petrolatum from the lithium as possible. Thus the weight of the piece of lithium was inaccurate.
• When I put the piece of lithium in the flask, some of the hydrogen gas produced from the reaction of lithium and water must have escape before I replaced the stopper. This source of inaccuracy can be reduced by replacing the stopper quickly before effervescence in the water starts indicating that the reaction has started.
• When collecting the hydrogen produced, there was a bubble of air in the measuring cylinder. Although I took this into account when recording the results, this factor will contribute to a measure of inaccuracy with the amount of gas collected.
• Some of the hydrogen produced would have dissolved into the water in the measuring cylinder thus the final amount of hydrogen collected would be lower than the actual amount. This source of inaccuracy can be reduced by using a different apparatus to measure the amount of gas produced like a syringe.

Experiment 2

• I used the solution from experiment 1 thus all the inaccuracies from experiment 1 would contribute to the overall inaccuracy in experiment 2. This can be reduced by making experiment 1 as accurate as possible or by using accurate industry manufactured lithium hydroxide solution.
• The hydrochloric acid I used was supposed to be a certain concentration. If however, its concentration wasn’t exactly as we were informed, then this would introduce a big inaccuracy in my experiment.
• Although the pipette is quite accurate, human error while measuring a specific amount of the solution from experiment 1 might make the reading inaccurate. This can be solved by practice on the part of the one carrying out the experiment.
• Titration of the solutions is also prone to human error. Practice can reduce this error margin n so can repeating the titration several times to find an average, which should be more accurate than the individual results.

While the lithium is stored, the surface will slowly react with air forming lithium oxide. This will have the effect of increasing the weight of the lithium because the relative atomic mass of oxygen is 16g mol-1, more than double that of lithium, 6.9g mol-1. Thus a lithium atom is much lighter than a lithium oxide molecule. This helps to explain why the average of the results for the Ar of lithium is higher than the values from the references.