2Li(s) + H2O(l) à 2LiOH(aq) + H2(g)
The mole ratio of lithium to lithium hydroxide is:
2:2
This means that the number of moles of lithium is equal to the number of moles of lithium hydroxide. So the number of moles of lithium is also 7.468 x 10-3.
Using the same equation as before:
Relative atomic mass = mass (g)
Number of moles
The mass of lithium used in 100cm3 is 0.05g.
So, the relative atomic mass of lithium = 0.05g
7.468 x 10-3
= 6.6952
Conclusion: The relative atomic mass of lithium is 6.70
EVALUATION
I feel that overall the results of my experiment were fairly accurate. I can test the accuracy by calculating the percentage of accuracy for each experiment. This is done by dividing the calculated result of the relative atomic mass by the actual atomic mass (6.9) of lithium, then multiplying this by 100:
Method 1: 6.64 x 100 = 96.2 % accurate
6.9
Method 2: 6.70 x 100 = 97.1 % accurate
6.9
This shows that my results have a high percentage of accuracy.
There was however an anomalous result in the titration in Method 2. It was very different from the other results so, for accuracy I ignored the anomaly when calculating the average titre. This anomaly may have been caused by a number of factors. The lithium is kept in oil while in storage to prevent it from oxidising. When using the lithium the oil must be cleaned off to ensure that it reacts to its full potential. If the oil is not cleaned of the lithium will not produce as much hydrogen, also if there is oil left on the lithium when weighing it may affect the weight, making the experiment less accurate. It would improve the accuracy and reliability of my experiment to completely clean of the lithium before using it, although care must be taken to make sure the surface of the lithium does not oxidise.
Another factor that may have caused the anomaly is error in measurement. This could be a misjudgement through human error for example; an error in misjudging the watermark in a burette or measuring cylinder could cause slight inaccuracy. The measuring devices that I used are all fairly accurate, the scales have an accuracy of ±0.005g and the measuring cylinder, burette and pipette have accuracies of ±0.05cm3, used accurately this equipment have sufficient accuracies for the purposes of my experiment. To ensure accuracy and reliability, measuring equipment should have the highest accuracy that is available and immense care must be taken when measuring. Another example of human error that may affect results is the decision of exactly when the end point of titration is and the reaction time between realising the end point and stopping the acid. This error can be minimised by taking care and slowing the acid towards the end point.
During Method 1, while calculating the number of moles of hydrogen it was assumed that the experiment was being carried out under standard room temperature and pressure, although this was not checked. To improve the accuracy of my results the room temperature should be checked. Because, if the temperature is not standard, one mole of the hydrogen would not take up 24dm3, which would make the calculations inaccurate.
According to the percentage accuracies, Method 1 is more accurate than Method 2. There are a number of factors that could be responsible for the lower percentage accuracy of Method 2; error in measurement of the lithium or distilled water, misjudgement of water mark in the measuring tube, oxidation of the lithium. Also, I repeated Method 2 three times, allowing me to disregard anomalies and take an average. But, because I had to use the solution from Method 1 in Method 2, I did not repeat it. Any error in measuring the lithium or distilled water or fault in the lithium during Method 1 is likely to affect the results of Method 2. This means that ideally the solution from Method 1 would have an accuracy as near to 100% as possible. This could be done by repeating Method 1, at least three times, each time keeping the solution. Calculate the atomic mass from each volume of gas produced, the solution with the highest accuracy can then be used for Method 2. The calculation of the atomic mass of lithium from Method 2 would then be expected to produce the highest accuracy and most reliable result, according to the calculated percentage accuracies.
Determination of the Relative Atomic Mass of Lithium - Planning
Lithium (Li), chemical element of Group 1a in the periodic table, the alkali metal group, lightest of the solid elements. The metal itself -which is soft, white, and lustrous- and several of its alloys and compounds are produced on an industrial scale.
Lithium reacts with water at room temperature to produce hydrogen gas and lithium hydroxide.
2Li(s) + 2H2O(l) 2LiOH(aq) + H2(g)
If a known mass of lithium is dissolved in water, the volume of hydrogen produced can be used to calculate the relative atomic mass of lithium. This will be the basic thesis for experiment 1.
The resulting solution of Lithium hydroxide can be titrated with a known concentration of hydrochloric acid to find its concentration, and this can be used to calculate the relative atomic mass of lithium. This will be the basic thesis for experiment 2.
LiOH(aq) + HCl(aq) LiCl(aq) + H2O(l)
Experiment 1
- I will arranged the apparatus as shown in fig. 1a
-
I will measure 100cm3 of distilled water using a burette - 2 x 50 cm3 and empty it into the conical flask. I will use a burette because it is very accurate as it is measured to the nearest 1.0 cm3 compared to a measuring cylinder, which is measured to the nearest 2cm3.
- I will then weigh a piece of lithium making sure that there is as little petrolatum in which it is stored as possible on it. I will put the lithium on a piece of paper and measure the weight. I will then remove the lithium n record the weight of the paper and the excess petrolatum. I will subtract this from the first weight reading to find out the weight of the lithium.
- I will then remove the stopper on the conical flask, add the lithium and quickly replace the stopper.
-
After the effervescence stops, indicating that the reaction is over, I will record the amount of gas collected in the measuring cylinder.
Experiment 2
- I will arrange the apparatus as shown in fig. 1b
- First, I will fill the burette with the standardised hydrochloric acid.
-
Then I will pipette 25cm3 of the solution from experiment 1 into a clean conical flask.
- I will add 5 drops of phenolphthalein indicator to the solution in the flask.
- I will titrate the solution with the hydrochloric acid and record the results.
- 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 titration’s 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-J
In 1 dm3 of base there is 0.0112 mol of Lithium Hydroxide
2 moles of Lithium produce 2 moles of Lithium Hydroxide = 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.
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
The relative atomic mass of lithium is 6.9g mol-1
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. Replacing the stopper quickly before effervescence in the water starts can reduce this source of inaccuracy.
- 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.
Determination of the relative atomic mass of lithium.
Method -Section 1 -Apparatus
Ø Conical Flask
Ø Bowl
Ø Measuring cylinder
Ø Clamp stand
Ø Rubber tubing
Procedure
I set up the apparatus. Then I carried out the correct measurements of water and lithium. I weighed the lithium as well, however when I did this I removed as much of the oil as possible, but not all of it. This was then placed in a small container. The container with the lithium inside was then placed in a conical flask, upright. A bung was placed on top to ensure the collection of gas was successful. I shook the conical flask so the container fell on its side to allow the reaction to take place. The gas produced was collected in a measuring cylinder.
Section 2 - Apparatus
Ø Conical flask
Ø Burette
Ø Clamp stand
Procedure
I pipette 25cm3 of the solution from section 1, which was then placed into a conical flask. 5 drops of phenolphthalein indicator was added to the solution. Hydrochloric acid was placed in the burette, which was titrated. I repeated the titration to ensure there was a fair test, these results were recorded.
Health & safety
Lithium metal is flammable and corrosive. Lithium reacts with water violently, creating a highly flammable gas Hydrogen. When the lithium reacts with water it also forming lithium hydroxide solution, which is corrosive. Lithium metal also burns in air. So therefore when using the metal lithium take great care, as you will need to wear goggles. Lithium is stored in oil, so when removing the oil wear rubber gloves to stop the metal reacting with any excess water on the skin. Hydrochloric acid is corrosive, so careful use is needed as well as wearing goggles.
Results - Section 1
It is assumed that 1 mole of gas occupies 24000cm3 at room temperature and pressure:
2Li (s) + 2H2O (l) 2LiOH (aq) + H2 (g)
Mass of lithium: 0.135g
Amount of hydrogen gas collected: 222cm3
Calculate the number of moles of H2:
Deduce the number of moles of lithium:
Relative atomic mass of lithium:
Section 2 - Titration results:
Average titration using 2 & 3:
41.2 + 41.3 = 82.5
82.5 / 2 = 41.25cm3
LiOH (aq) + HCl (aq) LiCl (aq) + H2O (l)
Number of moles of HCl used in titration:
Number of moles of LiOH used in titration:
(1:1, from equation)
Number of moles of LiOH present in 100cm3 in solution:
(Method 1)
Relative atomic mass of lithium calculated:
Evaluation
When comparing my result with the actual relative atomic mass of lithium (6.94), I find that my result is higher than that of the actual relative atomic mass. However I believe there are a few errors, which affect the accuracy of my result. When removing the oil from the lithium you cannot remove all of it, this could affect the weight of the lithium. If you could remove all of the oil this would increase the accuracy of the results. Another error was that the experiment was not fully carried out under standard conditions of 100kPa and 298K. To gain more accuracy I could have measured the pressure and temperature. Another error was that the tubing used was slightly permeable, which meant that H2 could escape to lower the volume of gas collected. To prevent this you could use a non-permeable tube, which would increase the accuracy of the results.