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Determination of the Relative Atomic Mass of Lithium

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Introduction

Determination of the Relative Atomic Mass of Lithium Results To determine the relative atomic mass of lithium I cut a piece of lithium, weighing 0.82g and reacted it with 100cm3 of distilled water. This reaction gives off hydrogen gas, which I collected in a 250cm3 measuring cylinder. I collected 129cm3 of hydrogen gas. To calculate the number of moles of hydrogen that I collected, assuming that 1 mole of gas occupies 24000cm3 at room temperature and pressure, I must divide the amount of gas I collected by 24000. 129 � 24000 = 0.005375 moles I can use this information to calculate the number of moles of lithium that reacted. I will multiply the number of moles of hydrogen I collected by 2 because of the 2:1 ratio of the lithium used to hydrogen given off. 0.005375 � 2 = 0.01075 moles Now, by dividing the amount of lithium I used in the reaction, which was 0.082g, by the number of moles of Lithium that reacted, 0.01075 moles, I can work out an estimate for the relative atomic mass of lithium. 0.082 � 0.01075 = 7.628 This is close to the actual relative atomic mass of Lithium but not as accurate as I had planned to get. After doing this I then titrated 25.0cm3 of the lithium hydroxide made by the reaction, with a 0.100 moldm-3 solution of hydrochloric acid. ...read more.

Middle

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. ...read more.

Conclusion

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. ?? ?? ?? ?? Rob Ayres ...read more.

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