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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 Results of first procedure: 158cm3 of hydrogen gas collected Results of second procedure Try Amount of acid needed (cm3) 1 39.20 2 39.30 3 39.30 Average 39.27 Conclusion Balanced Equation = 2Li(s) + 2H2O(l) � 2LiOH(aq) + H2(g) *All results will be rounded up to three significant figures Method 1 Assume that 1 mole of gas occupies 24000cm3 at room temperature and pressure. Firstly, the number of moles of Hydrogen gas needs to be calculated. This is done by dividing the volume of hydrogen gas produced in the experiment (This would be 158cm3) by the volume of 1 mole of hydrogen gas (24000cm3): Number of moles of H2 gas = Volume of H2 (g)/Volume of 1 mole of H2 (g) 158cm3/24000cm3 = 0.0065833 The result of this calculation is = 0.00658 The mole ratio of Lithium to Hydrogen is worked out by the balanced equation of the experiment, as shown above. For every 1 mole of hydrogen, there is going to be 2 moles of Lithium: 2Li(s) + 2H2O(I) � 2LiOH(aq) + H2(g) Mole ratio Li : H 2 : 1 For every 1 mole of hydrogen, there must be 2 moles of Lithium. Therefore, we need to multiply the number of moles of H2 gas to make the number of lithium and hydrogen moles equal. To multiply the number of moles of hydrogen gas, we need to times the number of moles of hydrogen gas by two: 0.00658 x ...read more.

Middle

Overall, the accuracy of measurements made isn't very precise as the human eye was used. If the measurements were taken by looking down at the apparatus, up at the apparatus or meeting it at the middle of the eye, each measurement could be slightly different. This method is called parallax. To make sure that the lithium had all traces of oil removed, we had to wipe the piece of lithium with filter paper. This method could have increased inaccuracy of the experiment, as all traces of oil might not have been removed. We had no indication of this; therefore, again we had to use the human eye. As determined above, the human eye isn't very precise, as it cannot determine the accuracy, in this case, if all traces of oil were removed from the lithium sample. Another inaccuracy that could have taken place in the experiment is the collection of hydrogen. In the experiment, the stopper in the flask, which was filled with 100cm3 of distilled water, was removed, lithium was added and the stopper was replaced as quickly as possible. Hydrogen would have been lost, in the time taken form adding the sample of lithium to the flask with distilled water and replacing the bung. Therefore, the final volume of gas evolved in the measuring cylinder would not have been correct; therefore, the titration experiment (second experiment) would have also been wrong as the same solution was used for the second experiment. ...read more.

Conclusion

Also to record and make note of the temperature of the room as an indication of what atmospheric conditions, the experiments are carried out at. Other sources or errors observed in the experiment are as follows. Firstly a major limitation of the experiment was using a measuring cylinder. This proved inaccurate, as hydrogen gas might have entered the cylinder even before the experiment had started making the results inaccurate. An improvement for this inaccuracy is to use a measuring cylinder. A measuring cylinder has measurements on it and is more accurate as the inside bung pushes itself outwards to indicate that gas is being collected. Also, the amount of gas collected is easier to measure as there are units on the cylinder. For stability and accuracy purposes, the measuring cylinder would have to be held horizontally using a clamp stand. Secondly, using a delivery tube would have been inaccurate, as gas might have got stuck in it. Because the tube was bent at various places, e.g. when it entered the measuring cylinder, this would have an impact made on the volume of gas collected, as not all of the gas would have been collected. A glass tube would have been more accurate to use, along with a measuring cylinder as this would be kept straight, and the bends made in the tube would be manufactured rather than produced by the person carrying out the experiment. The overall experiment involves weighting, collection of hydrogen and titration. Within the context of this experiment, the titration is not a significant procedural error. Faizah M (c)2003 Pg. 1 of. 4 ...read more.

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