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Determining the Relative Atomic Mass of Lithium.

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Introduction

Determining the Relative Atomic Mass of Lithium. To calculate the relative atomic mass, the moles of H2 produced when Li reacts with HCl. Attempt 1 Attempt 2 Attempt 3 Amount of H2 produced. (cm3) 181 174 178 On the first attempt 181cm3 of H2 was collected. The molarity of H2 must be found to enable us to calculate the relative atomic mass of lithium. 1 mole of gas occupies 24dm3 at room temperature and pressure. 1/24000 x 181 = 0.0075 Moles of H2 2Li(s) + 2H2O(l) --> 2LiOH(aq) + H2(g) The moles of Li that reacted must now be found. This result must multiply this result by 2 as Hydrogen as a gas is a diatomic particle (there are 2 atoms of hydrogen). = 2 x 0.0075 M = 0.0150M Therefore in 1g of Lithium reacts with H2O there is 0.015M moles of hydrogen given off. Relative Atomic Mass = Mass / Molarity Relative Atomic Mass = 0.1 / 0.015 = 6.66 The second attempt 174cm3 of H2 was collected. ...read more.

Middle

25cm3 of LiOH containing 0.00332 M was neutralised, so 100cm3 of LiOH must have 0.0133 Mol. In 0.1g of Li there are 0.0133 Mol. Relative Atomic Mass = Mass / Molarity Ar = 0.1 / 0.0133 = 7.52 Overall Average Ar Titration + Reaction 1/ 2 = Overall Average Ar 7.52 + 6.75 = 7.1 Evaluation The accuracy of my results were relatively good. The relative atomic mass of Lithium that I was aiming to achieve was 6.9. The average relative atomic mass that I achieved in the reaction experiment was 6.75, which is quite close to the actual Ar, on the second attempt the actual Ar was achieved this shows my results to be fairly accurate. However the majority of the experiments were either fractionally higher or lower. The titration experiment was further out than any of the reaction results; I think this is down to errors such as the response of the person carrying out the titration as more HCl than necessary may be added this is ...read more.

Conclusion

* Timing of the person carrying out the experiment, as there could be a loss of hydrogen just after it is added to the water, when the bung is not put on the conical flask quick enough. Alterations and improvements could have also been made to make the results more accurate, these are: - * The use of a gas syringe instead of a measuring cylinder to collect the hydrogen produced. This would be better as it a pocket of air can be left in the measuring cylinder when filling it with water, which would alter the reading. However there is a problem with using the gas syringe as it only holds a volume of 100 cm3, if the gas collected exceeds this you will not have a reading. Overall I consider my experiments to be a success as the results were within 0.2 of the actual Ar of Lithium even though there were inaccuracies within my results. There were slight inaccuracies with the titrating, but this is the most reliable way to carry out this experiment. Nathan Stokes 1 Lithium Coursework ...read more.

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