The relative atomic mass of lithium

The reaction will look like this:

LiOH(aq) + HCl(aq)------→LiCl(aq) +H2O

The titration was repeated to obtain consistent results.

• For the whole investigation, I must give full instructions for the practical procedure and this must be followed exactly. It is however my responsibility to follow suitable safety precautions and to organise our time appropriately.

Equipment:                                                                              

-Safety Goggles                                                                            

-2 conical flasks

       -Laboratory balance

       -Thread

       -Distilled water

       -Phenolphthalein indicator

 

Heath and safety:

-  Every chemical is a potential hazard to health, the degree of risk depending on its physical and chemical properties and the kind of exposure

                                                                                                                   

Knowledge how to dispose of waste and how to deal with spillages

Immediate clearing of spillages of any kind, wet or dry

• any splashes into the eye must be washed out continuously for 10-15 minutes

Before starting an experiment, it is essential to ensure that all chemicals not involved in the work have been removed from the vicinity

Safety glasses and dust masks MUST be worn when

Method-1

Finding the relative atomic mass of lithium by measuring the volume of hydrogen produced.

               Fig : below shows the practical work:  

1. The apparatus is set up as shown. Then exactly 100.0 cm3 of distilled water.

 

2.  a mass of about  0.10g of lithium is  weighted, then the exact mass of lithium is recorded  in the appropriate format

      3. The stopper is removed. And the lithium is added quickly replacing the stopper      

          fast.

     

     4. The gas involved is collected , then the final volume is recorded.

     5. The solution produced is kept for the second experiment 

Method -2

1)25.0cm3 of the solution is peptide into the conical flask from method 1 and 5 drops of Phenolphthalein indicator are added.

2) Titrate with 0.100mol dm3 HCl(aq)

3) Then the result are recorded in the appropriate format

4) The titration is repeated until consistent results are obtained; the results are then represented in the table.

5) Then the average titre is recorded.

                                              Results   

     

                                                       

                                         

I gathered a large enough range of results to make accurate observations from.  

The experiment was repeated until consistent results were obtained;

                                               Analysis

Calculation:

1st test- by measuring the volume of hydrogen produced

A mass of 0.07 of lithium was used, which gave 116Cm3 of H2

• Calculate the number of moles of hydrogen

116/24000 = 0.0048 mol of H2

2Li (s) + 2H2O(l) ---→2LiOH(l) + H2(g)

so,   2* 0.0048 = 0.0097mol of Li

using the formula to find the RAM of Li ------→  Mole = mass/Ar or Mr

so,  0.07/0.0097 = 7.2g  of Li

The answer is very close to the standard RAM of Li

Error calculations, 7.2*100/7=102.86 -   which means that there is 2.86% error in the first method

2nd test - by titrating the lithium hydroxide with HCl.

From the results table the mean titre is found by adding the runs altogether that divide the by 2

Mol =vol/1000*concentration

       = 38.4+35.7+38.6=56.35 /2 = 28.175cm3

       = 28.175/1000*0.100=0.002818mol

 

       = 100*0.002818/25= 6.21

Error calculations:

6.2*100/7= 88.57 ---- which means that there is 11.43 % error in the second method

   

      Conclusion:

After finishing the experiment, I came to the conclusion that that the value calculated, for the first method was more accurate than the value for the second one. This is not what I was expecting, because in the real sense of things the second method should be more accurate. This is due to the more precise equipment that was used in the second method, e.g. pipette, burette. This unexpected outcome could be explained in terms of experimental error.

I noticed that in the first method the experimental error was lower than that of the second method; this suggests that second method is more inaccurate than first one in my experiment. Therefore we had an unexpected outcome.  

Evaluation:

 

My method was good enough to make a conclusion, because i had i large range of results to make accurate observation from.

The experiment was successful overall based on the fact that the value in method 1 (7.2) and method 2 (6.21) are reasonably clause to the standard value of RAM of Li which is 7.OO .But during the experiment i encounter many problems, such as inaccurate mass balance, inpure Lithium ( which effected the experiment and maid unfair), unpractised apparatus and equipment which effected the experiment to surten extent etc.

The value calculated for method 2 was less accurate than the value in the method 1. this was not expected  because the apparature and the equipment (pipett and burett)  in method 2 are more precise then that of the method 1.

There were a lot of sources of errors in the experiment:

For example, The lithium was impure ( it had oxides and oils) which effected the experiment to some extend.

There was also volume floatation (escape) between the period from which the lithium was introduced into conical flask and the stopper was placed on it.

The measuring cylinder was not the precise apparatus  to measure the gas that evolved, so is counted as source of error because inaccurate readings means inaccurate results, which then effect the experiment.

The  mass balance despite taking quick readings, was not the most accurate possible. so it would make the experiment unfair.

The techniques used for the experiment were simple and almost accurate:

Te technique used in method 1 was easer compared to the technique used in method 2. This is because in method 2, titration was involved which is muche more difficult to perform.

Method one was performed faster then method 2, whether method 2 much slower because a lot of equipment were used such as pipette burette, which you need to take your time to be accurate.

The methods were very simple, but not detailed enough to make the experiment very accurate.

if the methods had more details in it then  this will inpruve the accuracy of the experiment.

The method didn’t give enough information of how to calculate the results, so i strugeled a bit, but in the end i worked out with the help of the book.

To minimise the errors the method should have explained in detailed the performers that the experimenter has to carry out.

To make the experiment the most accurate possible the equipment should be the more advanced ever, and variables should be pure with out any mixture.

For further improvement a freshly cut pure lithium should be used, this could be done by putting the lithium in the vacuum before transferring it into the conical flask. A freshly cut pure lithium means having accurate results (out comes).

Also after introducing the lithium into the conical flask the stopper should be replaced very quickly so a low minimum gas escapes. Then to measure this gas a more accurate apparatus  should be used like(Eudiometer ).The  mass balance despite taking quick readings, was not the most accurate possible, so to improve the accuracy of the experiment  a more advanced electronic mass balance should be used. I think more time is also needed. If i was doing the experiment again i would like a bite more time to perform the experiment as this would improve the accuracy of the experiment