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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 Planning Lithium (Li), chemical element of Group 1a in the periodic table, the alkali metal group, lightest of the solid elements. The metal itself -which is soft, white, and lustrous- and several of its alloys and compounds are produced on an industrial scale. Lithium reacts with water at room temperature to produce hydrogen gas and lithium hydroxide. 2Li(s) + 2H2O(l) 2LiOH(aq) + H2(g) If a known mass of lithium is dissolved in water, the volume of hydrogen produced can be used to calculate the relative atomic mass of lithium. This will be the basic thesis for experiment 1. The resulting solution of Lithium hydroxide can be titrated with a known concentration of hydrochloric acid to find its concentration, and this can be used to calculate the relative atomic mass of lithium. This will be the basic thesis for experiment 2. LiOH(aq) + HCl(aq) LiCl(aq) + H2O(l) Experiment 1 1. I will arranged the apparatus as shown in fig. 1a 2. I will measure 100cm3 of distilled water using a burette - 2 x 50 cm3 and empty it into the conical flask. I will use a burette because it is very accurate as it is measured to the nearest 1.0 cm3 compared to a measuring cylinder which is measured to the nearest 2cm3. ...read more.

Middle

and lithium-6 (7.5 percent) so the relative atomic mass of lithium is; (92.5 x 7) + (7.5 x 6) = 6.925g mol-1 100 From The periodic table in Cambridge Chemistry 1: The relative atomic mass of lithium is 6.9g mol-1 Results that form the graphs below are Data for Coursework with calculated values for Ar of Li from all students. These graphs below represent the dispersal of the calculated values for the relative atomic mass of lithium from experiment 1 and 2. By looking at the graphs above, it is evident that the majority of values for the Ar of lithium are between 6 and 8 For an average of all the results, I will add all the values together then divide the product by the number of values. The average of the results including the anomalous ones in experiment 1 gives the value for the Ar of lithium as 7.22 and the standard deviation which describes the spread of the data is 1.03. This shows that the spread of the data is low and that the data is precise. The average of the results including the anomalous ones in experiment 2 gives the value for the Ar of lithium as 7.79 and the standard deviation is 1.21. This shows that the spread of the data is low but higher than that of experiment 1. ...read more.

Conclusion

This source of inaccuracy can be reduced by using a different apparatus to measure the amount of gas produced like a syringe. Experiment 2 * I used the solution from experiment 1 thus all the inaccuracies from experiment 1 would contribute to the overall inaccuracy in experiment 2. This can be reduced by making experiment 1 as accurate as possible or by using accurate industry manufactured lithium hydroxide solution. * The hydrochloric acid I used was supposed to be a certain concentration. If however, its concentration wasn't exactly as we were informed, then this would introduce a big inaccuracy in my experiment. * Although the pipette is quite accurate, human error while measuring a specific amount of the solution from experiment 1 might make the reading inaccurate. This can be solved by practice on the part of the one carrying out the experiment. * Titration of the solutions is also prone to human error. Practice can reduce this error margin n so can repeating the titration several times to find an average, which should be more accurate than the individual results. While the lithium is stored, the surface will slowly react with air forming lithium oxide. This will have the effect of increasing the weight of the lithium because the relative atomic mass of oxygen is 16g mol-1, more than double that of lithium, 6.9g mol-1. Thus a lithium atom is much lighter than a lithium oxide molecule. ...read more.

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