Determination Of The Atomic mass of Lithium.

The equation for the neutralisation reaction in your titration is shown below

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

 

• Calculate the number of moles of hydrochloric acid used in titration.

Concentration =  No. of moles

                    Volume dm3

1. =   Moles

          0.04375

Moles = 0.1 * 0.04375

Moles = 0.004375 mol dm3 

• Deduce the number of moles of lithium hydroxide used in the reaction.

The ratio of Lithium hydroxide to Hydrochloric acid is 1:1

So the number of moles used in the titration reaction is the same 0.004375 mol dm3 

Calculate the number of moles of Lithium Hydroxide present in 100cm3 of the solution from method 1.

0.004375 * 4 = 0.0175

• Use this and the original mass of lithium to calculate the relative atomic mass of lithium.

Ar = Mass

        Moles

Ar = 0.13

        0.0175

Ar = 7.42857

Evaluation:

The actual relative atomic mass of lithium is 6.94.

• I think that the overall accuracy of my experiment had room for improvement. The result from experiment 1 was very accurate as the relative atomic mass equated to be 7.09 which is very close to the actual atomic mass of lithium. As for the result from method 2 the mass that I calculated was 7.42 which is quite far off from the real mass.
• So I found that method 1 was 2.1% off the real atomic mass. Method 2 was 6.5% off the real atomic mass. So the second method was a great deal less accurate below in the following bullet points I will explain where I think the errors that could have made this inaccuracy happened.
• Firstly I will analyse where I think that errors which would have affected my result in method 1:

• The bung had to be secured over the conical fast quickly but it’s almost impossible to do this without losing any hydrogen.
• The tube had to be in the measuring cylinder. (This wasn’t an error which would affect your results, just ruin your experiment as no hydrogen would be collected).
• When you placed the measuring cylinder in the trough, the water line had to be at a mark on the scale so you could accurately determine the end volume of hydrogen produced with accuracy.
• Lithium had to be weighed using appropriate scales.
• Tube had to be checked to see if there were any holes or leakages so no hydrogen would be lost.
• Impurities may be present in the lithium distorting the results slightly. (if there is any impurities it would only be very small amounts)
• All traces of oil may not be removed from the lithium before it is reacted with the water.
• The age of the lithium may be a cause for slight error in the results as older lithium may have a thicker layer of metal oxide around it.

• Now I will explain ways to ensure the most accurate results in method 1.

• Secure the bung in the top of the conical flask as fast as you possibly can, making sure that it is totally secure in the flask.
• Check before you add the lithium to the water that the tube is inside the measuring cylinder securely.
• When measuring the lithium on the filter paper, place the filter paper on the scales zero them then weigh the lithium to make sure it’s accurate, Do this using scales which weigh to 0.001g to ensure total accuracy.
• Put your thumb over one end of the tube and blow in the other to make sure there are no holes in the tube where hydrogen could leak out.
• Remove as much oil as possible before adding the lithium to the water.

• Secondly I will analyse where I think that errors which would have affected my result in method 2:

• The inaccurate measuring of the lithium hydroxide solution.
• All lab equipment for example conical flasks had to be totally clean any chemicals that could have been in the containers before could contaminate the results. (The Phenolphthalein indicator could be incorrect if contaminants were present)
• When titrating, the volume needed to totally neutralise the lithium hydroxide was unknown so when using the burette sometimes I could have used more acid than was needed to neutralise the solution.
• The solution changed from purple/violet to colourless when the PH was neutral it was difficult to know exactly when the solution was totally neutral just by judging from the colour.
• The acid had to be mixed perfectly so that it was the exact concentration needed.

• Now I will explain ways to ensure the most accurate results in method 2.

• Use the equipment as best you can measure the solution carefully to ensure accuracy.
• Ensure the acid is mixed correctly to produce the exact concentration needed.

• Now I will explain which experiment was more accurate and why I think this is the case.

• There is one point that has to be remembered. The second experiment contains the errors that I have made in the first experiment, any errors from the first experiment are carried over making the titration experiment seem a lot more inaccurate, and this is shown by the results that I have obtained. Also the fact that there are three titration experiments making three results and I used the average of these this backs up the fact that the second method is less accurate.
• The first experiment shows a far more accurate result than the second, I think this is due to the fact that as you can see from the section above I have found a lot more ways to make the first experiment more accurate, plus
• Also as we are only in a school lab the precision of the equipment cannot be of an exact standard, in the two methods you have to measure out volumes of solutions so again slight errors will occur.