Deduce the number of moles of magnesium, which reacted
Number of moles of Magnesium = Mass / Ar
= 0.12 / 20.27
= 0.00592 moles
Calculate the relative atomic mass of magnesium
RAM = Mass / Number of Moles
= 0.12 / 0.00592
= 20.27
Evaluation
There are certain errors within the experiment that may partially account for the unreliable results. But I will talk about that in the errors section. There are certain changes that I would make to this experiment. Firstly I would use a gas syringe to increase the accuracy of the readings. I would also try and do something to stop the gas escaping before the bung was securely fixed. This may have been something to do with the errors. I have come up with a suitable solution for this:
Also I think that the rubber stoppers were not entirely fixed well. I think that some gas may have been able to escape. For this error I think that I would use ground-glass stoppers. I would also use a burette instead of a measuring cylinder so that I was more accurate when measuring HCl.
Also, some gas may be given off before I have had the time to place the cork in the flask. This would mean that I lose some volume of gas. The magnesium may have been impure and the oxide layer may have taken some of the mass. I also found that when I was trying to weigh my magnesium the balance was wandering and I had to pick a reading and this may not have been the most accurate. Also when the bung is pushed into the flask it may displace some air and create an error.
Further Work
If I was going to do further work I might use some different metals. I might also use different acid concentrations and different acids altogether. Also I might like to find the relative atomic mass of a metal using a more accurate method and achieve more accurate results. I would use the method of some kind of electrolysis.
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 (24.1) of magnesium, then multiplying this by 100:
Method 1: 20.27 / 24.1 x 100 = 84.1 % accurate
Method 2: 20.27 / 24.1 x 100 = 84.1 % accurate
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 water mark 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 disreguard 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.
