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# relative atomic mass of litium

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Determination relative atomic mass of lithium

Evaluation

Identify calculated error

Any experiment can not in 100% accurate. Some errors are known and I can be calculated (e.g. the accuracy of the apparatus we use).

Calculated errors for my experiment are listed below:

Pipette: ±0.06ml

0.06/25=0.0024=0.24%

Burette: ±0.05ml

0.05/42.50=0.00283=0.12%

Balance: ±0.005g

0.005/0.13=3.8%

Measuring cylinder (250ml): ±2ml

2/206.70=0.9%

Measuring cylinder (100ml):±1ml

1/100.00=1%

Identify way to improve my calculated errors

From calculated errors above we can see that the measuring cylinder (100ml) was a relatively big error in calculated error.  Its percentage error is 4 times larger than the error on pipette and 8.3 times larger than the burette this could be improved my using a more accurate volumetric flask (±0.06ml). This could dramatically minimize my most significant error from 1% to 0.06% according to my calculation below.

0.06/100=0.06%

Also the measuring cylinder (250ml) I have used to collect hydrogen was also a significant error. I could use a more accurate gas syringe instead, but we do not have gas syringe which capable of collecting 206.70cm3 gas. Therefore it is not practical.

Identify the most significant calculated error and ways to improve

The balance I used was the most significant error. It is 3.8%, it is 15.8 times greater than the calculated error on pipette and 31.6 times greater than the error on burette This could be improved by using a more accurate balance. E.g. a balance that’s accurate to 3 decimal places. This could decrease my error margin dramatically from 3.8% to 0.38%according the calculations below:

Balance: ±0.0005g

0.0005/0.13=0.38%

The other way of minimize the error is to use a larger quantity of lithium. (E.g. use 1.3 grams of lithium)This could decrease my error margin dramatically from 3.8% to 0.16% according the calculations below:

Balance: ±0.005g

0.005/1.3=0.16%

But if 1.3g of lithium was used there will be 1.3/6.9=0.188mole of lithium, according to ‘1 mole of any gas at room temperature and pressure occupies 24dm3’(from book ‘Calculation for chemistry’), there will be 4512cm3 of hydrogen will be produced, and we have no measuring equipment to measure this vast amount of gas therefore this method is not practical.

Identify Procedure errors and ways to improve

I have used the displacement of water to measure the amount of Hydrogen produced. I have noticed that it is almost impossible to fill up the measuring cylinder completely, there is always about 1cm3 of gas left in the gas cylinder. This was one of my most significant errors in my procedure. Also the measuring cylinder is only accurate at 20 degrees, but the temperature of the class room is impossible to control, therefore I could not be sure if the measuring cylinder is at its most accurate states. Both of the problems could be resolved by using a gas syringe.

When I was collecting the gas produced I need to put the lithium in before I seal it, there is about half of a second when the gas is not collected. the way I can minimize the time duration when gas is not collected is to hold the rubber bung as close to the top of the conical flask and to get another person to drop in the lithium metal into the flask and then place the bung in position as quickly as possible.

Lithium is a reactive metal it reacts when in contact with air (oxygen), therefore it needs to be stored In oil. I need to remove all the oil before I can weigh it in order to weigh it out accurately. I can not removing the oil completely by using the procedures stated in the planning (wipe oil off using paper). I can improve this by dipping the lithium in alcohol in order to remove the oil completely. But If I remove the oil, as I was transferring the lithium from the balance into the conical flask it forms an oxidised layer this could also cause an incomplete reaction with water. The best way to do it is to complete this whole weighing out and transfer of lithium process in an inert atmosphere, this could prevent the formation of the oxidise layer.

Reliability of my result

Determine the relative atomic mass of lithium was done by two methods. First it is done by measuring the gas produced. I do not think my results from this part of the experiment are reliable, because of all the procedure errors and in accurate measuring cylinder mentioned above. Also I have only done it once, therefore I have to other result to compare it with, and it could even be an anomalies value. The second method was titration. This method was more reliable than the first method because I have completed the experiment three times and all my result agrees with in 1d.p.

Further investigation

In the future I would also like to try using different method such as measuring the gas produced.

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