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Determination of the relative atomic mass of Lithium.

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Introduction

Chemistry Coursework Stephanie Knowles Determination of the Relative Atomic Mass of Lithium Analysing: We followed the plan set to us and, for method one, attempted capturing the hydrogen gas produced by the reaction of lithium and water. We decided to do this four times to gain more reliable results, and to identify any anomalies. On our first attempt we used 0.10g of lithium, this amount produced 90cm� of hydrogen gas, on our second attempt we used 0.9g of lithium, which produced 130cm� of hydrogen. At this point it was clear that there was a mistake as bigger quantities of lithium would have reacted more, producing more gas, or at least the amount should have been around the same as our first attempt. On the third try we used 0.13g of lithium- this reaction caused 230cm� of gas to be collected. Making the conclusion that one of our 0.9g or 0.10g of lithium results was incorrect, we attempted a 0.10g of lithium again this produced 250cm� of gas, again not agreeing with our result for the 0.13g of lithium. ...read more.

Middle

We need to know how many grams produces 1 mole, therefore divide 1 mole by 0.020mols: 1mol - 0.020mols = 5g Giving the answer that the relative atomic mass of lithium, is 5. Procedure Two- Calculate the number of moles of HCl used in the titration- We can do so using this balanced equation: LiOH (aq) + HCl (aq) LiCl (aq) + H O (l) 12.6 cm� (average titration amount) The HCl has a concentration of 0.1mol dm�. We must use this formula- 12.6 1000 x 0.1 = 0.00126 Therefore 0.00126mols HCl were used. Deduce the number of moles of LiOH used- From the above equation we can see that the same amount of HCl is used as LiOH so there is also 0.00126mols of LiOH used. Calculate number mols LiOH present in 100cm �of the solution from procedure one- We know that there was 0.00126mols present of LiOH in 25cm�, to calculate how much was present in 100cm�, multiply by 4: 4 x 0.00126mols = 0.005mols Calculate the relative atomic mass of lithium- The relative atomic mass is the same as 1mole of lithium. ...read more.

Conclusion

Other then human error the only explanation for this huge difference was the lithium we used. Lithium must be stored in oil as after a minute or so in the presence of moisture and oxygen the surface becomes tarnished to a grey colour and perhaps the occurrence of lithium oxide as well. This might well have affected the collected lithium hydroxide in the conical flask therefore affecting the titration. Slicing an entirely silvery coloured fresh piece of lithium and using it instead of a piece from the outside of the lithium 'block' could have prevented this. In conclusion, the experiment was not a successful one- the apparatus problems and the lithium used (which would have also explained the diverse amounts of hydrogen collected in procedure one) would explain the very different results of the relative atomic masses from each other and the correct one. With the adjustments suggested mad the results would have been more reliable and closer to the truth. ...read more.

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