Saturday, May 14, 2011

Describe of experiment in Paper 2 (Essay)

Form 4: Chapter 3: 

Describe an experiment in the laboratory to determine the empirical formula of magnesium oxide. Your answer should consist of the following.

  • Procedures of the experiment
  • Calculation involved
  • Precautions need to be taken
Given that relative atomic mass of Mg = 24,   O = 16       
     [13 marks]

Suggested Answer:

[Material and apparatus ]
[Magnesium ribbon, sand paper, crucible with lid, tongs, Bunsen burner, tripod stand, pipe-clay triangle and balamce.] or  [diagram]  2    
·          A crucible and its lid is weighed and its weight is recorded.        1
·          A 10 cm length of magnesium ribbon is coiled loosely and is placed in the crucible.                                                        1
·          The crucible with its lid and content are weighed again and the weight is recorded.                                                                   1
·          The crucible is heated strongly without its lid.                             1
·          When the magnesium starts to burn, the crucible is covered with its lid.   1
·          Using a pair of tongs, the lid is lifted at intervals.                          1
·          When the burning is completed, the lid is removed and
the crucible is heated strongly for 2 minutes.                               
·          The rucible is allowed to cool to room temperature.                     1
·          The crucible and its lid and content are weighed again and its weight is recorded.                                                                        1
                                                                                                            max 5
[Precautions taken]                         
·          The magnesium ribbon is cleaned with sand paper to remove the layer of magnesium oxide on the surface.
·          The crucible must cover with its lid to prevent the white fumes of magnesium oxide of escaping.            
·          The processes of heating, cooling and weighing are repeated until a constant mass is obtained.                              [any two of the above]                              2
[ Results ]                                                                        
Crucible + lid                                          =   a  g         
Crucible + lid +  magnesium                    =   b  g         
Crucible + lid +  magnesium oxide          =   c  g                     1
[ Calculation ]                                                                    
Number of moles of atoms
Simplest ratio of moles
Empirical formula =  MgxOy                                                       2+1

No comments:

Post a Comment