Determination of the relative atomic mass of Lithium using Titration.

Diagram for method 1

        

        

Result table

Calculation

Assuming that I mole of gas occupies 24 000cm3 at room temperature and pressure

2Li(s) +  2H20(l)                        2LiOH(aq)  +  H2(g) 

(2 moles)                                         (1 mole)

(44.8dm-3)                                       (24 000)

2Li  :  H2  

Therefore the ratio is 2 : 1

1. No. Of moles of hydrogen = 140/24 000

                                             = 0.0058 moles

2. No. Of moles of Lithium = 0.0058 * 2

                                                      = 0.0117 moles        

Mass of lithium = 0.10g

Moles of lithium = 0.0117

Moles = Mass / Mr

3. Relative atomic mass of lithium  =  

  0.0117 = 0.10 / Mr

 0.0117Mr = 0.10

 Mr = 0.10 / 0.0117

                                                                      Mr = 8.55

Method 2

1. Set up the equipments as stated below.

2. Using pipette 25.0 cm3 of the solution in the conical flask from method 1 into a clean 250 cm3 conical flask and add 5 drops of Phenolphthalein indicator.

3. Titrate with 0.100mol dm3 Hydrochloric acid.

4. Record the results in an appropriate format.

5. Repeat the titration to obtain consistence results. Show all of the results.

 

6. Record the average titre.

Diagram for method 2

        

Hydrochloric acid

     

                     

Lithium hydroxide and

Phenolphthalein indicator

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Results Table

Calculation

LiOH(aq)       +       HCl(aq)                             LiCl(aq)     +     H2O(l)

1. Moles of HCl used in titration

         Moles   =  molarity   x    volume

                      = 0.100   x   (42.7/1000)

                      = 0.00427

2. No. of moles of LiOH  =  0.00427 as the ratio is 1:1.

3. No. of moles of LiOH present in 100cm 3 of the solution from method 1 = 25 /100

        = 4

     Therefore 0.00427 x 4 = 0.01708

4. Relative atomic mass of Lithium = 0.100 / 0.01708

     = 5.85

Final Result table

Conclusion

I have found that the relative atomic mass of lithium in method 1 was 8.55 but when it was titrated in the method 2 the relative atomic mass was 5.85.

Therefore when the lithium hydroxide was titrated with hydrochloric acid the relative atomic mass was decreased to 2.7.

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Evaluation

The method used was not simple to the investigation that I was carrying out because it requires accuracy and it was too complicated, as I had to do the both method. First I had to collect the hydrogen gas then clear the equipments and then again had to set the equipments for the second method, which was titration.

 

Then I had to titrate it drop by drop to see the colour changing to colourless.

I had to titrate it carefully to get the sufficient results that were needed.

I had to take three readings to get the sufficient mean titration result, which required a lot of time to do this.

Accuracy

By taking results for three times allowed me to gain a good average of the concentration. Also the use of three results meant that the reliability and the accuracy of my results were high.

 

Problems encountered

The main problem was that it was difficult to put the measuring cylinder full of water into the plastic box of water up side down to collect gas.

The second problem was that I did not rinse the conical flask every time after each titration therefore more titration was required. And the titre readings were not similar.  

Improvements

By looking at the above problems I would use the gas syringe to measure the hydrogen gas. I would rinse the flask out properly to ensure that it is fully cleansed and ready for the next part of the investigation.

Extension

As an extension to the investigation I could use other elements, to calculate the their relative atomic mass and compare them.