Determination of the relative atomic mass of lithium.

> Clamp stand

Procedure

I pipette 25cm3 of the solution from section 1, which was then placed into a conical flask. 5 drops of phenolphthalein indicator was added to the solution. Hydrochloric acid was placed in the burette, which was titrated. I repeated the titration to ensure there was a fair test, these results were recorded.

Health & safety

Lithium metal is flammable and corrosive. Lithium reacts with water violently, creating a highly flammable gas Hydrogen. When the lithium reacts with water it also forming lithium hydroxide solution, which is corrosive. Lithium metal also burns in air. So therefore when using the metal lithium take great care, as you will need to wear goggles. Lithium is stored in oil, so when removing the oil wear rubber gloves to stop the metal reacting with any excess water on the skin. Hydrochloric acid is corrosive, so careful use is needed as well as wearing goggles.

Results

Section 1

It is assumed that 1 mole of gas occupies 24000cm3 at room temperature and pressure:

2Li (s) + 2H2O (l) 2LiOH (aq) + H2 (g)

Mass of lithium: 0.135g

Amount of hydrogen gas collected: 222cm3

Calculate the number of moles of H2:

Deduce the number of moles of lithium:

Relative atomic mass of lithium:

Section 2

Titration results:

Titration

Volume of HCl (cm3)

.

42

2.

41.2

3.

41.3

Average titration using 2 & 3:

41.2 + 41.3 = 82.5

82.5 / 2 = 41.25cm3

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

Number of moles of HCl used in titration:

Number of moles of LiOH used in titration:

(1:1, from equation)

Number of moles of LiOH present in 100cm3 in solution:

(Method 1)

Relative atomic mass of lithium calculated:

Evaluation

When comparing my result with the actual relative atomic mass of lithium (6.94), I find that my result is higher than that of the actual relative atomic mass. However I believe there are a few errors which affect the accuracy of my result. When removing the oil from the lithium you cannot remove all of it, this could effect the weight of the lithium. If you could remove all of the oil this would increase the accuracy of the results. Another error was that the experiment was not fully carried out under standard conditions of 100kPa and 298K. To gain more accuracy I could have measured the pressure and temperature. Another error was that the tubing used was slightly permeable, which meant that H2 could escape to lower the volume of gas collected. To prevent this you could use a non-permeable tube, which would increase the accuracy of the results.