to determine the relative atomic mass of lithium. We will be doing this via two methods:

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AS – Assessed Practical (Skills I, A and E) - Determination of the Relative Atomic Mass of Lithium

23rd March 06

The aim of this experiment is to determine the relative atomic mass of lithium. We will be doing this via two methods:

  • The first method will be to collect the gas evolved when lithium is reacted with distilled water and consequently calculate the relative atomic mass.
  • The second method will be to calculate the relative atomic mass through titration with Hydrochloric Acid (HCl).  

Method 1 Results:

The volume of hydrogen collected from the reaction of 0.099 grams of lithium with 100.0cm3 of distilled water was 178 cm3.

Treatment of results:

We assume that 1 mole of gas occupies 24000 cm3 at room temperature and pressure.

The chemical equation for this reaction is:

2Li(s) + 2H2O(l) → 2LiOH(aq) + H2(g)

We must firstly calculate the number of moles of Hydrogen produced using the following equation:

Moles of gas =     Volume (in cm3)

                               24 000

Therefore,

Moles of hydrogen  =     178     =  7.4167 x 10-3   moles

                                     24 000

According to the mole ratio in our chemical equation, 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 = 7.4167 x 10-3 moles x 2

                                              = 0.014833 moles

Now that we have obtained the number of moles of lithium, we 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 of:

Relative Atomic Mass =       Mass (g)

                                        Number of moles

The mass of lithium used is 0.099g so to find the relative atomic mass of lithium:

        

Relative atomic mass =     0.099     = 6.6742

                                                   0.014833

So the Relative Atomic Mass of Lithium is 6.67 (to 3 significant figures).

Method 2 Results:

We titrated 10.0cm3 of the product solution (LiOH(aq)) from method 1 with 0.100 mol dm-3 HCl. The results of this titration were:

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  • Concordant Results

On average, 25.0 cm3 of LiOH(aq) required 14.80 cm3 of 0.100 mol dm-3. This value was obtained by using our three concordant results.

Treatment of results:

The chemical equation for this reaction is:

LiOH(aq) + HCl(aq)  LiCl(aq) + H2O(l)

To find the Relative Atomic Mass of Lithium, we must first calculate the number of moles of HCl used in the titration. This can be done by using the following equation:

Number of Moles = Concentration x Volume = 0.100 x 14.80 = 1.480 x 10-3 moles

                                  ...

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