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Determining the relative atomic mass of lithium

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Introduction

Assessed practical- Determining the relative atomic mass of lithium o Hazards of chemicals used and obtained: Method 1- 2Li(s) + 2H2O(l) � 2LiOH(aq) + H2(g) Method 2- LiOH(aq) + HCl(aq) � LiCl(aq) + 2H2O(l) In method 1 hydrogen gas is produced. This is flammable and although not a lot will be produced it would be safe to keep the product away from open flames, sparks, etc. The product of the second method, lithium hydroxide, is alkaline and an irritant. When handling it care should be taken and if it is spilt all of it should be cleaned up immediately. Although not hazardous to us the lithium is very reactive and is used in small quantities. Also precaution must be taken in order to get accurate results, as with the distilled H2O that may become contaminated. In method 2 hydrochloric acid is used and although it only has a concentration of 0.1mol dm-3 care must be taken so it isn't spilt or brought into contact with skin, as it is an irritant. ...read more.

Middle

So; moles of HCl used in the titration = concentration � volume = 0.1000mol dm-3 � 36.03 dm3 1000 = 3.603�10-3 moles As 1 mole of LiOH reacts with 1 mole of HCl, [LiOH(aq) + HCl(aq) � LiCl(aq) + 2H2O(l)] then 3.603�10-3 moles of LiOH were used in the titration. Only 25cm3 of the LiOH solution at a time during the titration, so the number of moles in the 100cm3 solution from method one = 4 � 3.603�10-3 = 0.0144moles. Therefore, the relative atomic mass of lithium = moles = 0.1000g = 6.94g mol-1. 0.0144dm3 o Evaluation: Overall my results are quite accurate and in the case of method two correct, when rounded to three significant figures. In my calculations I used four significant figures in order to improve accuracy, and by using the average of three titrations (all within one decimal place of each other) ...read more.

Conclusion

Ideally a polarized, un-reactive solution should be used instead. Another measurement error is the resolution of the apparatus used; the scales measured to the nearest 10mgram, the burette measured to the nearest 1cm3 whilst the measuring cylinder to the nearest 2cm3. With a higher resolution method two will have more accurate results. The most significant measurement error is the amount of gas given off in method one. Aside from the procedure the measurement can be modified to reduce the error: H2 gas produced = 172cm3 � 1cm3 (as the resolution is to the nearest 2cm3). Minimum possible value: 171cm3. Relative atomic mass of lithium: 171/24000 = 7.125�10-3moles. 7.125�10-3 � 2 = 0.01425moles. 0.1/0.01425 = 7.02g mol-1(3s.f). Maximum possible value: 173cm3. Relative atomic mass of lithium: 173/24000 = 7.208�10-3moles. 7.208�10-3 � 2 = 0.0144moles. 0.1/0.0144 = 6.94g mol-1(3s.f). ROB HENDERSON ...read more.

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