Percentage composition by mass of magnesium oxide

Procedure:        

1. Using a balance, the mass of a clean, dry porcelain crucible and lid were determined.
2. The magnesium ribbon was rubbed with the steel wool and was folded so that the ribbon could fit into the bottom of the crucible.
3. The mass of the crucible, lid and magnesium ribbon were determined.
4. The crucible was placed securely on the clay triangle.  The lid was set slightly off centre on the crucible to allow air to enter but to prevent the magnesium oxide from escaping.
5. The lab burner was places under the crucible, lighted and heated with a gentle flame.
6. Gradually, the flame intensity increased until all the magnesium turned into a white powder.
7. The flow of gas was cut off and the crucible, lid and contents, were allowed to cool down.
8. Using the stirring rod, the contents of the crucible were crushed into a powder.  10mL of distilled water was added to the powder.
9. The crucible and its contents, with the lid slightly ajar, were heated gently for 3 minutes and then strongly for 7 minutes.
10. The crucible and contents were allowed to cool.
11. Using the balance, the mass of the cooled crucible, lid and contents were determined.

Observations:

Calculations:

1.        No. of moles of Mg   =    Mass of Mg         =    0.13g    =  0.00534mol of Mg

                                Molar mass of Mg          24.3g

No. of moles of O   =     Mass of O             =    0.08g    =  0.005mol of O

                                Molar mass of O              16g

        Ratio of magnesium atoms to oxygen atoms is   Mg : O :: 1.06 : 1

2.        Total mass of Mg + O  =  Mass of Mg + Mass of O

                                 =  0.13g + 0.08g

                                 =  0.21g

        % of Mg in MgO  =  Mass of Mg    x   100   =   0.13g   x   100   =  62%

                                 Mass of MgO                      0.21g

% of O in MgO  =  Mass of O    x   100   =   0.08g   x   100   =  38%

                              Mass of MgO                   0.21g

Discussion::

4. Use your evidence to calculate the percentage composition by mass of magnesium oxide.

The percentage composition (calculated above) by mass of magnesium oxide is 62% magnesium and 38% oxygen.

f)        If some of the MgO had escaped from the crucible, would your percentage composition calculation of magnesium be too high or too low?  Explain.

        If some of the magnesium oxide had escaped from the crucible, the percentage composition calculation of magnesium would be too high because some of the oxygen, since it a gas, may have escaped from the crucible.  The percentage composition calculation of magnesium would the increase because of the decrease of oxygen.

g)        If the magnesium had reacted with some other components in the air, would your percentage composition calculation of the magnesium be too high or too low?  Explain.

        If the magnesium had reacted with some other components in the air, then the percentage composition calculation of magnesium would be too low because some of the magnesium had already been reacting with other components and leaving a smaller amount of magnesium to react with oxygen.

h)        The magnesium ribbon was polished to remove any white film on its surface before beginning the experiment, explain why this is necessary.

        The magnesium ribbon was polished to expose the shiny part because this is the part of the magnesium that is reactive.

Sources of Error::

• We only got to do the lab once because our magnesium ribbon took long to turn into white powder, so therefore, our results are not as accurate as possible.
• The crucible may not have been thoroughly cleaned.
• The flame was turning yellow on the Bunsen burner.

Conclusion::

        The percentage composition calculation for the magnesium in magnesium oxide, when calculated, worked out to 62% and the percentage composition calculation for the oxygen in magnesium oxide, when calculated, worked out to 38%.  My hypothesis was that the magnesium would be 60% and the oxygen would be 40%.  Therefore, my hypothesis was very close.  In this lab, I learned how to calculate the percentage composition calculations and I can know relate the calculations to a real experience, which would make the concept of percentage composition more straight-forward and easier to understand.