Average Volume of Hydrogen Gas: (144+139+172)/3 = 152ml.
Processing of Results:
2Li(s) + 2H₂0(l) 2LiOH(aq) + H₂(g)
No. of moles of H₂ = volume/(24x1000) = 152/24000 = 0.00633
No. of moles of Li = 2 x 0.00633 = 0.01266 (ratio of lithium to hydrogen is 2:1)
Relative atomic mass of lithium = mass/number of moles = 0.1/0.001266 = 7.89 (3 sf)
Uncertainties:
Uncertainty in mass of Li = 0.01/0.1 x 100% = 10
Uncertainty in volume of H₂ = 1/152 x 100% = 0.657
Total uncertainty = 10 + 0.657 = 10.7% approx
Thus, relative atomic mass of Li = 7.89 to 3 sig fig +/- 10.7%
= 7.89 to 3 sig fig +/- 0.8
The literature (true) value of relative atomic mass of lithium is given as 6.94 so the percentage error in the result is as follows:
We can conclude that the experiment gave an inaccurate value for the relative atomic mass of lithium, since the percentage uncertainty is quite high and there is a significant difference between the calculated value and literature value of lithium. This shows that the experiment had systematic errors which caused this inaccurate value.
Do the uncertainties in the apparatus account for this percentage error? No, if we take into account the uncertainty of +/- 0.8, the experimental relative atomic mass value ranges from 7.09 to 8.69. This range does not incorporate the true value of the relative atomic mass of lithium, so we can see that there must be other random or systematic errors in the experiment.
Possible sources of error:
The reason why the amount of hydrogen collected wasn’t enough (it was expected that the volume of hydrogen would be around 200cm³) was due to the fact that once we added the small piece of lithium in the flask of water, the bung that was meant to cover the flask was not placed as soon as the lithium was added. This is because of a slow reaction time and the fact that the bung didn’t fit through the opening of the flask at the exact moment. A less amount of hydrogen gas was collected therefore decreasing the number of hydrogen molecules and therefore a less number of lithium molecules which lead to a smaller calculated value for the relative atomic mass. A method to try and avoid this error could be by tying a piece of string around the lithium and placing it in the flask with your partner at the ready to place the bung on top.
A systematic error was the fact that different balances were used to measure the size of the beaker containing lithium. With each experiment a different value of mass for the beaker was obtained each time. The usage of different balances caused an imprecise set of results for the mass of the lithium therefore concluding that the same amount of lithium was not used for each experiment. Using the same balance for each experiment would successfully provide a more reliable set of results.
To avoid random errors, repeating the experiment is a key factor to obtaining accurate results. However, due to time constraints it wasn’t possible for me and my partner to repeat the experiment with relatively the same masses of lithium to get a more precise average for a more reliable result.
Human error and parallax error also play a part when acquiring an accurate result. Parallax error occurred when reading of the measuring cylinder to record the start and end volume to find out the volume of hydrogen gas. The clamp stand which was used to hold the measuring cylinder placed in the tub of water was not able to hold the measuring cylinder straight, therefore making it difficult for us to read the value of the volume since the meniscus was tilted. Human errors include the inability to measure the exact amount of lithium for each experiment. The apparatus such as the balance also gave different answers, questioning how dependable our final results are. It was also difficult to fill the measuring cylinder with water to the top so the start volume of water would be different for each experiment, therefore giving imprecise records.
by
rimarif (student)
