An Investigation into the Atomic Mass of Lithium

An Investigation into the Atomic Mass of Lithium

There are two methods that can be used to determine the atomic mass of

a Group 1 metal. These are best described by the reactions that take

place during these methods.

Method 1:

2Li(s) + 2H20(l) è 2LiOH(aq) + H2 (g)

For the first reaction, the equipment must be set up as detailed in

the attached sheet.

A mass of lithium must be taken and weighed, the results recorded.

This mass is then to be added to the test tube, and the bung replaced.

As is shown by the reaction for the experiment, a gas (Hydrogen) is

released. This hydrogen displaces the water in the cylinder, and can

be measured.

The equation also shows that there is a relationship between the mass

of lithium added to the test tube and the total volume of gas that is

released by the reaction. The number of moles of lithium present in

the mass used must be half that of hydrogen in the gas collected in

the measuring cylinder.

Since at standard conditions a mole of gas occupies a specific volume

(22.4dm3) the number of moles of hydrogen collected can be worked out

from the volume of water displaced.

RESULTS

Initial measurement of volume: 0 cm3

Final measurement of volume: 58 cm3

Weight of Lithium: 0.03g

As there are always 22.4dm3 to a mole of gas at standard conditions,

there are 58/24000 moles of Hydrogen present in the 58cm3 space

displaced by the reaction.

2 moles of Lithium are required to produce 1 mole of Hydrogen, so in

order to produce 58/24000 moles of hydrogen, there must have been

58/12000 moles of lithium present in the mass used for this

experiment.

The mass used weighed 0.03g. As this is the weight of 58/12000 moles

of Lithium, 1 mole must weigh

(12000 / 58) X 0.03 g = 206.9 X 0.03 g = 6.2g

Method 2:

LiOH(aq) + HCl(aq) è LiCl(aq) + H20(l)