Determination of the relative atomic mass of lithium.

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Determination of the relative atomic mass of lithium

The aim of this experiment is to determine the relative atomic mass of lithium. I will be doing this in two ways: the first method will be to collect the gas evolved when lithium is reacts with distilled water and calculate the relative atomic mass this way. The second method will be to calculate the relative atomic mass through titration.

ANALYSIS - Method 1

The volume of hydrogen collected from the reaction of 0.05 grams of lithium with 100.0cm3 was 90.3cm3.

The chemical equation for this is:

2Li(s) + 2H2O(l) 2LiOH(aq) + H2(g)

The mole ratio of lithium to hydrogen is:

2:1

First I must calculate the number of moles of hydrogen, I can do this using the following equation:

Number of moles of gas = volume

24 000 (one mole of any gas has a volume of 24 dm3 RTP)

Therefore, the number of moles of hydrogen = 90.3cm3

24 000cm3

= 3.7625 x 10-3 moles

According to the mole ratio, there are twice as many moles of lithium than hydrogen present. Therefore the number of moles of lithium equals the number of moles of hydrogen multiplied by two:

Number of moles of lithium = 3.7625 x 10-3 moles x 2

= 7.525 x 10-3 moles

Now that I have calculated the number of moles of lithium, I can now calculate the its relative atomic mass using the following equation:

Number of moles = mass (g)

Relative atomic mass

This equation can be rearranged to make the relative atomic mass the subject:

Relative atomic mass = mass (g)

Number of moles

The mass of lithium used is 0.05g.

To find the relative atomic mass of lithium:

Relative atomic mass = 0.05g

7.525 x 10-3

= 6.6445

Conclusion: The relative atomic mass of is 6.64

Method 2

I titrated 25.0cm3 of the solution from method 1 with 0.1M hydrochloric acid. The results of this titration are:

  • 18.5cm3
  • 18.7cm3
  • 20.1cm3
  • 18.8cm3

I treated the 20.1cm3 result as an anomaly, as it was very different from the other results. So I will not be using it in my average titre.

Average titre = 18.67cm3 (to 2 decimal places is sufficient accuracy)

The chemical equation for this reaction is:

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

The mole ratio of lithium hydroxide to hydrochloric acid is:

1:1

To find the relative atomic mass of lithium, I must first calculate the number of moles of hydrochloric acid used in the titration. This can be done by using the following equation:

Concentration (M) = number of mole

Volume (dm3)

The volume of hydrochloric acid (dm3) = 18.67cm3

1000

= 0.01867dm3

The concentration of hydrochloric acid is 0.1M

The above equation can be rearranged to make the number of moles the subject:

Number of moles = concentration (M) x volume (dm3)

This gives:

Number of moles of HCl = 0.1M x 0.01867dm3

= 1.867 x 10-3 moles

I can now deduce the number of moles of lithium hydroxide using the mole ratio. The mole ratio of lithium hydroxide to hydrochloric acid is 1:1. This means that the number of moles of lithium is the same as the number of moles of hydrochloric acid.

So, the number of moles of lithium hydroxide is 1.867 x 10-3

I now need to calculate the number of moles of lithium hydroxide present in 100cm3 of the solution from Method 1. I used 25cm3 of the solution from Method 1. Since 25cm3 fits into 100cm3 four times, to find the number of moles of lithium hydroxide present in 100cm3, I will need to multiply the number of moles of lithium hydroxide in 25cm3 by four:

Number of moles of lithium hydroxide in 100cm3 = 1.867 x 10-3 moles x 4

= 7.468 x 10-3 moles

Using this result I can now calculate the relative atomic mass of lithium. For this I must go back to the chemical equation from Method 1:

Join now!

2Li(s) + H2O(l) à 2LiOH(aq) + H2(g)

The mole ratio of lithium to lithium hydroxide is:

2:2

This means that the number of moles of lithium is equal to the number of moles of lithium hydroxide. So the number of moles of lithium is also 7.468 x 10-3.

Using the same equation as before:

Relative atomic mass = mass (g)

Number of moles

The mass of lithium used in 100cm3 is 0.05g.

So, the relative atomic mass of lithium = 0.05g

7.468 x 10-3

= 6.6952

Conclusion: The relative atomic mass of lithium is 6.70

EVALUATION

I feel that overall the results of ...

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