The equation for the neutralisation reaction is as follows:
LiOH(aq) + HCl(aq) → LiCl(aq) + H2O(l)
I will now calculate the number of moles of hydrochloric acid used by multiplying the average amount of HCl used in dm3, by the concentration of it, which is 0.100 moldm-3
Average amount of HCl used in dm3 = 0.02253 dm3
0.02253 × 0.100 = 0.002253 mol
So the number of moles of LiOH used is also 0.002253 moles because of the 1:1 ratio.
To calculate the number of moles of LiOH present in 100 cm3 of the solution from method 1, I must multiply the number of moles of LiOH present in the 25cm3 sample I used, which was 0.002253, by 4.
0.002253 × 4 = 0.009012 mol
I can now use this result and the original mass of Lithium that I used in my experiment to calculate the relative atomic mass of Lithium, using the equation - mass ÷ mole.
0.082 ÷ 0.009012 = 9.098
This result is not as accurate as I would have liked to get because the actual relative atomic mass of lithium is 6.9
Chemicals
This is a list of Chemicals I used in the experiments:
- Lithium
- Lithium Hydroxide
- Hydrochloric acid
- Phenolphthalein indicator
The chemicals I used in this experiment were quite hazardous so there were certain precautions I had to take.
Lithium is at the top of group 1 so although it is not as reactive as the other metals in that group, it still reacts vigorously with water. Because Lithium reacts with air, it had to be kept in oil to stop it reacting. I also had to wear plastic gloves when handling it, and use tweezers to pick it up, to stop it reacting with the moisture on my skin.
When Lithium reacts with water it makes Lithium Hydroxide, which is an alkali. Although it was not a very strong alkali, because of the low concentration, I was careful not to get it on my hands.
To neutralise the Lithium Hydroxide in the titration, I used 0.100 moldm-3 Hydrochloric acid. This is also not very strong, but can cause irritation to the skin and eyes, so I was again, careful not to get it on my hands. In order to stop the chemicals going in my eyes or on my clothes, I wore an apron and goggles through the whole experiment.
The last chemical I used was the Phenolphthalein indicator, which I used to see when the chemicals had neutralised. This is not a high hazard, but can stain my clothes so I wore the apron and made sure not to get it on my hands.
Evaluation
Overall I am quite pleased with my experiments, but I would have liked them to be more accurate. Before I started the experiments I rinsed out all my apparatus with distilled water to get rid of any substances that could have been in them. These are the factors which I think affected the accuracy of my experiments. Firstly, when I was doing method 1, there were a few areas where I think I lost some accuracy. When I was filling up my conical flask with 100cm3 I could have used less or more than exactly 100cm3, so the amount of water I was reacting with the Lithium could have been more or less than I thought, although I am sure I measured this very accurately so there should be little error there. Also the oil the Lithium was kept in may have still been on the lithium, or the lithium could have reacted with the air before I added it to the water, this could have slowed the reaction.
When I had to fill up the 250cm3 measuring cylinder with water to collect the hydrogen gas in, there was a bubble at the top, which I was unable to get rid of. This means that when I collected the hydrogen gas in the measuring cylinder, there was more gas in there than there should have been, but it was a small amount of air so the bubble shouldn’t have made too much of a difference.
When I added my lump of Lithium to the distilled water, it began reacting straight away so a small amount of gas would have escaped from the conical flask before I had a chance to put the top on.
When I did my titration there were some areas where I could also have lost accuracy. Firstly, when I was filling my pipette with 25cm3 of the Lithium Hydroxide I made, the pipette filler I used was not working properly so I had to use another, but I still found it difficult to get exactly 25cm3, also, I could have over or under filled the burette with hydrochloric acid, or over shot when I was titrating. However, the results I got from my titration are all within 0.1cm3 of each other so I am pleased with the accuracy I achieved here.
I think that my titration was much more accurate than the results I got from when I collected the gas. This is because there were more places where I am sure I made mistakes when I was collecting the gas, than when I was titrating the Lithium Hydroxide and also my results from my titration are all very close. I am sure that for both methods I measured out the chemicals with a high degree of accuracy, making sure that the bottom of the meniscus was on the line at eye level when measuring, It was only when collecting results from method 1 that I made mistakes. I think the most successful part of my experiment was my titration where I managed to get very accurate results.
To minimise error, I should have taken much more care in method 1, when I was collecting the gas. This is where I got the inaccuracy of my results. I could have put the top on the conical flask faster to reduce the amount of gas that escaped. Also, to increase the reliability of my results I could have asked someone to check my measurements to make sure I was reading them correctly.
A way of minimising errors caused by escaping gas could be to use a gas syringe instead of collecting the gas in a measuring cylinder, because there would be less error from bubbles in the measuring cylinder and the results would be much more accurate.
If I were to repeat the experiment I would take much more care when carrying out the experiments and collecting the results, but overall the experiment went as well as I expected.