To determine the relative atomic mass of Lithium

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PATRICK OLANIYI

 CHEMISTRY ASSESSED PRACITICAL

 

                                                            ANALYSIS

Aim -:  To determine the relative atomic mass of Lithium

 Lithium is a very reactive metal. In order to determine the relative atomic mass of Lithium, I will be doing it in two ways, namely-:

  1. By measuring the volume of hydrogen gas produced
  2. By titrating the produced LiOH with HCl

To measure the volume of Hydrogen gas produced, I reacted some Lithium metal with distilled water, but firstly I set up the apparatus I’ll be using, as illustrated in the diagram below.

When measuring the Lithium, safety was an important criterion for me because lithium is a very reactive metal with make cause burns. Firstly I took a clean conical flask of 250cm3, then I took a 100cm3 pipette and rinse it with some distilled water and pipette 100cm3 form the same distilled water into the clean 250cm3 conical flask. I then inserted a stopper that has a delivery tube in it. Later on I took a bowl and fill it up with water but the water was not full in the bowl. I collected 250cm3 of water from the bowl using a measuring cylinder. I then passed on the end of the delivery tube to the mouth of the measuring cylinder, which is filled with water.

       

           After setting up the apparatus, I went on to measure my Lithium metal. The Lithium metal was kept in oil because of it reactive nature. Before measuring the Lithium’s mass, I cleaned up the oil in because it might affect the rate of reaction. Once the oil is totally dried from the Lithium metal I took the measurement, which was 0.11g.

   I then went on to put the Lithium into the distilled water in the conical flask. Immediately when the Lithium was put in the distilled water, I inserted the stopper so that they will not be any lose of Hydrogen gas. I took the measurement of the hydrogen produced and it was exactly 190cm3. The remaining substance in the conical flask is Lithium Hydroxide, which I’ll be using for the next way to find the relative atomic mass of Lithium.

 The balanced equation below shows the reaction between Lithium and distilled water.

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

Since I know the volume of Hydrogen gas produced I can work out the number of moles of Hydrogen in the reaction, knowing the number of moles of Hydrogen gas produced can enable me to work the number of moles of Lithium in the reaction. That is

  1. mole of gas occupies 24dm3 or 24 000 cm3

    Therefore 190 cm3  =          190 /  24000  =  0.0079167 moles of H2

          Looking at the stoichiometry ratio of H2 : Li

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                                                 1 : 2

   That is twice of mole of Lithium is in the reaction.    

                           0.0079167 x  2   =  0.015833 moles of Li.

   n  = m / M

 Number of moles =  Mass / Molar mass

 Molar mass  =  Mass  / Number moles

                       Where mass of Li = 0.11g

                         Number of moles Li = 0.015833

  1. /  0.015833  = ...

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