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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 Reacting Lithium with water Mass of Lithium reacted 0.08g Volume of Hydrogen collected 131cm3 Titration Rough 1 2 Volume of LiOH 25cm3 25cm3 25cm3 Volume of HCl required 25.6cm3 25.4cm3 25.4cm3 Analysis 1. 2Li(s) + 2H20(l) 2LiOH(aq) + H2(g) a) Finding the number of moles of Hydrogen: To find the number of moles of Hydrogen, we use the values we have collected and place them into the ideal gas equation: n = ? P = 1.01 x 105 Pa = 101 000 Pa V = 131cm3 = 131 x 10-6 m3 = 0.000131m3 R = 8.31 J mol-1 K-1 T = 18�C = 291K n = PV = 101 000 x 0.000131 = 13.231 = 0.00547 mol (3 s.f) RT 8.31 x 291 2418.21 b) Finding the number of moles of Lithium: To find the number of moles of Lithium, we use the mole ratio found in the equation above: Mole ratio Li : H2 = 2 : 1 Therefore, nLiOH = 2 x 0.00547 = 0.0109 mol c) Finding the relative atomic mass of Lithium: n = 0.0109 mol m = 0.08g Ar = ? n = m therefore Ar = m = 0.08 = 7.34 (3 s.f.) ...read more.

Middle

mass of lithium used: 0.07g Therefore maximum percentage error for the balance in method 1 = 0.01 x 100 = 12.5% 0.08 250cm3 measuring cylinder in method 1: maximum amount of gas collected: 136cm3 minimum amount of gas collected: 126cm3 Therefore maximum percentage error for the measuring cylinder in method 1 = 5 x 100 131 = 3.8% Pipette in method 2: maximum amount of LiOH used: 25.05cm3 minimum amount of LiOH used: 24.95cm3 Therefore maximum percentage error for the pipette in method 2 = 0.05 x 100 = 0.2% 25 Burette in method 2: maximum amount of HCl used: 25.75 + 25.55 + 25.55 = 25.62 3 Minimum amount of HCl used: 25.45 + 25.25 + 25.25 = 25.32 3 Therefore maximum percentage error of the Burette in method 2 = 0.15 x 100 = 0.59% 25.47 Balance from method 1 for method 2: 12.5% Total maximum percentage error for method 1 = 3 + 12.5 + 3.8 = 19.3% Total maximum percentage error for method 2 = 0.2 + 0.59 + 12.5 = 13.29% Evaluation 1. Consistency of Titration results: All three of my titration results were consistent, and there were no anomalous results, with a difference of 0.2 between my rough titration and the other two experiments, which were the same. ...read more.

Conclusion

This is added to by the width of the meniscus, and the difficulty in having an accurate reading. Another possibly source of error is the time taken after adding the lithium to the water to replace the bung. Although I tried to limit this time, some lithium could have reacted and produced hydrogen gas before I managed to replace the bung. However, my method was very accurate, which is shown by the accuracy of my results. 5. Improvements to minimise sources of error: To minimise the error from using the measuring cylinder, I could use a gas syringe instead, which is more accurate, and would allow me to calculate a more accurate result for the volume of hydrogen produced. This would also discard the error involved with inverting the cylinder of water into the water bath, resulting in the bubble of air forming. It is difficult to minimise the sources of human error, such as the readings from the burette, or the time between adding the lithium and replacing the bung. However, by ensuring that I carry out the experiment as accurately and replacing the bung as swiftly as possible helps to minimise these errors. ?? ?? ?? ?? Gemma Edney Chemistry Coursework 1 October 2007 1 ...read more.

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