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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 Introduction In this experiment I will determine the relative atomic mass of lithium by two different methods: * By measuring the volume of hydrogen produced * By titrating the lithium hydroxide produced Results: METHOD 1 Assume that 1 mole of gas occupies 24000cm3 at r.p.t. 2Li(s) + 2H2O(l) LiOH(aq) + H2(g) Calculating the Ar of Lithium To find the moles of Lithium (Li) we must first find the moles of Hydrogen (H2) by using the formula: Moles = volume 24000cm3 Moles = 206cm3 24000cm3 Moles = 8.583 x 10-3 Now that we can determine the moles of Lithium (Li) by using the ratio between them: Ratio = Li : H2 2 : 1 ?Moles of Lithium = 2 x (8.583 x 10-3) = 0.017166666666 Now that we know the number of moles of Lithium we can determine its relative atomic mass (Ar). ...read more.

Middle

This is because the agreed Ar for Li is 6.9 % difference (6.9 - 6.55) x 100 = 5 % 6.9 My experiment was not 100% accurate due to the following errors: Points to note: * The measuring cylinder is not very accurate because the error here can be �2cm3. A gas syringe is more accurate to measure the volume of hydrogen gas; here the error could be �1cm3. * The gas that had been collected is not at atmospheric pressure (1 atm). However 1 mole of gas can only occupy 24000cm3 at r.t.p. Errors in Method 1 (collecting gas) * A 25cm3 measuring cylinder was used to measure the volume of gas. However a measuring cylinder is only accurate to % error = 2 x 100 = 0.97 % 206 * A 100cm3 measuring cylinder was used to measure the volume of distilled water. Here the error is �1cm3. ...read more.

Conclusion

% error = 0.1 x 100 = 0.4 % 25 * The accuracy of a burette is �0.05cm3. My average titre was 41.05cm3. % error = 0.05 x 100 = 0.1 % 41.05 Overall % error for method 2 = 0.1 + 0.4 = 0.5 % Overall % error = 0.5 + 10.97 = 11.47% Improvements for Method 1 * To prevent any H2 gas escaping the apparatus below could be used: To start the reaction, the conical flask would need to be tilted so that the water goes inside the test tube. * A gas syringe is more accurate in measuring the amount of gas. The accuracy is �1cm3. * Also a gas syringe should be used to eliminate any possibility of H2 gas dissolving in the water. The apparatus below could be used for above improvements: * To make sure the pressure of the gas is at 1 atm, lower the measuring cylinder until water level, inside and outside are the same. Improvements for Method 2 * If I had more time I could've done another titration so that the average would be more accurate. ...read more.

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I used 25cm3 of the solution from Method 1. Since 25cm3 fits into 100cm3 four times, to find the number of moles of lithium hydroxide present in 100cm3, I will need to multiply the number of moles of lithium hydroxide in 25cm3 by four: Number of moles of lithium hydroxide in 100cm3 = 1.867 x 10-3 moles x 4 • Over 160,000 pieces
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