Relative atomic Mass of Lithium

Tino Chingwaru

Determination of the relative atomic mass of Lithium

Evaluation

I was given a task in which I was had to investigate and determine the relative atomic mass of Lithium. I was given the opportunity to use two different methods of which I could use to get my final result.

The practical takes a vast amount of concentration and there are a number of measurement errors that can occur while carrying out the practical. Problems that affected the measurement such as; the equipment being used, not being 100% accurate when measuring out the substances being used, which usually gave an outcome of the readings given being slightly higher or slightly lower than the amount needed. This is evident in the use of the pipette, which expands and contracts due to the temperature and surrounding, so to get what is seen as an accurate reading would be nearly impossible. This degree of uncertainty is referred to as the tolerance. The tolerance could be a problem when the measurements are carried forward to the calculations, for example; when calculating the number of moles of Lithium Hydroxide in the second method. The 100cm3 of water measurement from method 1 would be used to work out the concentration, so an error could just affect the results.

These results are the tolerance values for each piece of equipment used within the experiment.

10cm3 Pipette- (+ or -) 0.04cm3
100cm3 Pipette- (+ or -) 0.15cm3
250cm3 Measuring cylinder- (+ or -) 1cm3
Burette- (+ or -) 0.04cm3
Balance- (+ or -) 0.005g

The equipment used within this experiment played a major role, and so whenever making a measurement I always considered the fact that any errors in my measurements would have an effect later on during my investigation.

When using the 10cm3 pipette, the values are always rounded up or down due to ...

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These results are the tolerance values for each piece of equipment used within the experiment.

10cm3 Pipette- (+ or -) 0.04cm3
100cm3 Pipette- (+ or -) 0.15cm3
250cm3 Measuring cylinder- (+ or -) 1cm3
Burette- (+ or -) 0.04cm3
Balance- (+ or -) 0.005g

The equipment used within this experiment played a major role, and so whenever making a measurement I always considered the fact that any errors in my measurements would have an effect later on during my investigation.

When using the 10cm3 pipette, the values are always rounded up or down due to the fact that the minimum value that can be measured when the bottom of the meniscus is on the line is 9.96, where the maximum is 10.04.

The following calculation shows how the significance of the errors is calculated in relation to comparing the measurement errors;

Uncertainty (%) = Multiplied by 100

Below are all the measurement errors of all relative pieces of equipment used during the experiment.

Pipette (10cm3)

Uncertainty (%) = Multiplied by 100

Pipette (100cm3)

Uncertainty (%) = Multiplied by 100

Measuring Cylinder

Uncertainty (%) = Multiplied by 100

Burette

Uncertainty (%) = Multiplied by 100

Balance

Uncertainty (%) = Multiplied by 100

From my results it is clear that the important area of measurement source is the balance, this is due to the fact that it has the highest uncertainty.

From my calculations I also saw that even though the tolerance of the measuring cylinder is greater to that of the pipette and the percentage uncertainty is lower which is due to the fact that the volume that has been measured in the measuring cylinder is larger than the pipette, meaning the measurement error produced by the pipette is more significant.

There are two ways in which you can minimise the measurement errors; one being to measure a larger amount, but by doing this I may need to alter my method, which can be done by either using alternative equipment such as a larger burette or increased concentration of Lithium Hydroxide solution. Another way can be to attempt to use a piece of equipment with a lower tolerance value, but this is not always possible.

Having worked out all the percentage uncertainties of all my measurements I can now identify the balance as the most significant measurement error as it has the largest uncertainty.

Another thing that can be minimized within this experiment is the procedure errors. Which are errors that are obtained from errors in the method; one way in doing so would be when adding the Lithium to the water, when doing so the hydrogen gas would have escaped before I managed to secure the stopper and this could lead to me having major inaccurate results from the measure of gas produced. To minimize this error I could have attempted to secure the stopper much quicker but this wouldn’t have a great deal of effect. I could have also stuck a piece of Lithium on the bottom of the stopper so my results are not lowered.

Another example of a procedural error is due to the fact that the oil on the Lithium may not have been totally cleaned off as all we had to get rid of this was the filter paper, and due to the excess oil this could lead to there being a higher mass within the Lithium, therefore the calculated RAM would be relatively lower than expected. To prevent this from happening I could have used an inert solvent to wash all the oil off the Lithium providing me with all the correct results.

After carrying out and completing my experiment, and going through two methods in which to get the result of my task, I can see why it is essential to have more than one method, as one method will not show the mistakes and effectiveness another method will.

I feel that the most effective method was method 2. This is because it was most effective in finding the relative atomic mass of lithium. I feel this is because the most inaccurate piece of equipment, which is the balance, was not included in this method which would have gave a better chance of getting accurate results. Another reason is that within this method the Burette was used which had the most accurate functions to get the most reliable results, which put this method at a steady advantage as the least accurate apparatus was excluded and the most accurate was included.