To find the relative atomic mass of a sample of Lithium
When carrying out this practical wear safety glasses at all times.
Where the rubber gloves at all times and make sure there are no holes or tears.
When filling the burette hold the funnel slightly above the rim so as to break the seal allowing air to leave and fluid to replace it instead of the funnel filling up.
The above helps to prevent you over filling the burette and it spilling over. Always fill the burette after moving it lower down so that if it does spill over it does not pour onto you or into your eyes.
Wash your hands thoroughly after handling any Acids or Bases.
Do not rub your eyes or put your hands or fingers in your mouth at any point during the experiment.
Do not sit at the desk as this may prevent you moving in the event of a spillage.
Lithium is a highly reactive element and must be treated with caution, it will react simply with the water that is in the air. This is why it is stored in oil to prevent it reacting. The problem with Lithium coming into contact with the skin is that it will react with the water in our skin and then form highly concentrated acids as well. This is why rubber gloves must be worn at all times.
HCl is a strong acid and must be treated carefully because contact with the skin could cause irritation and severe damage if it isn’t washed off.
If the skin does come into contact with either of theses then it must be washed thoroughly and immediately.
;50 cm3 Burette.
;25 cm3 Pipette.
;Boss clamp and Stand.
;Distilled water dispenser
;250cm3 Measuring cylinder.
Carry out the method set out for the first Experiment being careful to follow the instructions carefully.
Using the following formula and the results I gain from titrating Lithium Oxide and Hydrochloric Acid I hope to find the relative atomic mass of the LI sample.
Ma x Va = Mb x Vb
Avagadro’s number is more commonly known as the mole. It is the number of C12 atoms in 12 g. This is 6.23×10 -23 atoms. By dividing the mass in grams of a substance by the Molecular mass Mr you will find the number of moles of that substance you have.
No of Moles = Mass(g) ï¿½ Mr
Molarity is the number of moles of a substance that is dissolved in 1 litre of water or a decimeter3.
Molarity = Moles(m) ï¿½ Vol (dm3)
Titration is the exact neutralisation of an Acid and a Base in order to find an Unknown (ie Molarity, No of moles etc). By using a Burette and an Acid Base indicator, an exact amount of reactant 1 can be added to a known amount of reactant 2 until a colour change of the indicator indicates the solution has become neutral.
Weight of Li=0.10grms
Volume of H2 gas produced=203cm3
The results I gained I compared directly to the value of the relative atomic mass of Lithium given on the Periodic table. I got the following percentage errors.
Experiment 1= Experiment 2=
This showed me that experiment 1 was more accurate than experiment 2. There were some problems which had an effect on both experiments, the Li sample we used was not completely pure and there will have been some impurities in it. When the samples were weighed because they were coated in oil some oil was measured as well but I tried to minimise this by soaking up some of the oil with tissue paper before weighing. If I had soaked all of the oil of the Li would have immediately reacted with the surrounding water in the air and created a coating of Li oxide on the outside of the sample therefore again affecting the results.
In experiment 1 the method had some very major inaccuracies, like the loss of gas when the Li sample was dropped in because it would be very difficult to put the bung on before it hit the water. This was what I accounted for as the main inaccuracy in experiment 1. However the experiment was still more accurate than the titration. The method of this experiment could have been improved. My suggestion would be to place the Li in an empty flask and then inject through a second glass tube and in air tight conditions the 100cm3 of water. Like shown below. This would minimise the amount of gas lost in the addition of the Li sample.
By using a more accurate method of measuring the collected gas a better result could have been achieved because the 250cm3 measuring cylinder was not very accurate.
In experiment 2 there were also some sources of error. But I got three results within 0.02cm3 of each other which leads me to believe that there was a major source of error in the first part of the experiment most likely the preparation of the Li sample and it’s weighing. I must also account for the impurity of the sample, the second experiment was a titration which is generally very accurate.
The Molarity of the HCl was measured to .001 mol/dm3.
The volume of HCl added from the burette was measured accurate to 0.1cm3 at 20?C.
The mass of Lithium solid measured was accurate to .01 grams.
The 25cm3 pipette measures exactly at 20?C.
When measuring liquids the meniscus must be taken into account, due to surface tension the surface within the pipette burette etc is curved. The meniscus is always lined up with the bottom touching the line and the line being held at eye level so that it is a solid line and not a cylinder like you would see if it was above or below eye level. This is why there is a continuous line around the glassware.
When using a burette so long as the meniscus is measured from the bottom at both the start and end point it will not effect the accuracy unlike in a pipette.
When filling the burette always remove the funnel because a drop of reagent could fall into the burette altering the results because a burette can measure accurately enough for 1 drop to make a difference.
Other sources of error could be the amount of indicator used so I used 5 drop for each titration. Bubbles in the end of the burette could account for any inaccuracies and during the second titration I thought I had reached the end point but when I swirled the solution more and washed the flask sides with distilled water I found the solution turned purple again.
Overall the second experiment was much more accurate then the first shown by the difference in percentage error. However both had in built inaccuracies in the form of the oil on the Li sample and the purity of the sample.
I have found several things which could account for the inaccuracies in the second experiment. The Lithium must have been cut many times and so exposing quite a lot of its surface to the air for a short period of time causing the surface atoms to react with the surrounding air. The oil was also part of the measurement of the weight however I dried most of it off but this again would have caused some of the surface to have come in contact with the air. The indicator I used was Phenol Phtalein however I believe that this was not the best indicator to use for this titration. An indicator which changes under slightly acidic conditions is the best one to use when titrating a weak base. So an indicator like Methyl red may have been more appropriate.
The main problem with this experiment is not knowing exactly how much Lithium Hydroxide was made. Although the scales were very accurate, up to 0.01 grams. The Oil and the reactivity of the Lithium made it difficult to find.
One way this could be improved would be to Dissolve some Lithium in a known weight of water filter off the oil and reweigh the LiOH solution accounting for the lost Hydrogen. However this would be difficult to achieve when adding such small amounts of Lithium as 0.1 gram.
The first experiment although it had more obvious sources where error could occur turned out to be more accurate than the titration, which was inaccurate due to a combination of the above problems. The second experiment method could also have been improved by dissolving the LiOH solution by adding another 25cm3 so that there was plenty of solution to perform 4 titrations.
When I analysed the results I gained by using the Mr of Li from the periodic table I found that the either the true mass of the Li was 0.12 grams or that the volume of HCl that I neutralised the solution with should have been 36cm3 which are too very large errors. I feel that repeats of this experiment would be useful to see if it was just human error that caused the problem, or whether the wrong indicator could account for the excess of HCl added.
I thought the result I gained was not satisfactory and the margin of error for both experiments was too high.