Percentage composition by mass of Magnesium oxide

Topics: Chemistry, Magnesium, Magnesium oxide Pages: 8 (1458 words) Published: May 28, 2005

The purpose of this lab was to test the law of definite proportions for the synthesis reaction of combusting magnesium. In this lab, the polished magnesium ribbon was placed in covered crucible and was heated in order for it to react with Oxygen presented in air and in water provided. The result showed that Magnesium oxide formed through chemical reaction was made up of 60.19% magnesium and 39.81% oxygen, which is approximate proportion of both particles in every Magnesium oxide compound. From this lab it can be concluded that the law of definite proportion stating that the elements in a pure compound combine in definite proportion to each other is factual.


The law of definite proportion states that every chemical compound contains definite and constant proportion (by weight) of its constituent elements. The purpose of this lab was to test for law of definite proportion through determining percentage composition of each element in Magnesium oxide. I hypothesized that the composition mass of magnesium will be more than oxygen and this would be the result for other groups as well. My null hypothesis was that if the experiment is done accurately, the percentage composition of magnesium shouldn't exceed 65%.


-Refer to lab manual on page 180 in Nelson Chemistry 11.

-The following revisions were made:

-Nickel crucible and lid was used instead of porcelain crucible and lid

-Tripod was used instead of retort stand, ring stand and clamp


1. The nickel crucible and lid were cleaned and dried.

2. The observations about appearance of magnesium ribbon were made.

3. The magnesium ribbon was polished and rolled using pen in order to fit it in crucible.

4. As directed the mass of the crucible, magnesium ribbon and lid was measured. The total mass of all tree components together was measured as well.

5. The crucible and magnesium ribbon were placed on clay triangle on tripod stand. The lid was placed on crucible slightly off the center.

6. To give it start while waiting the small Bunsen burner was placed under the crucible and it was heated with a gentle flame.

7. The small Bunsen burner was then replaced with bigger burner to speed up the reaction rate.

8. The flow of gas was cut of when the magnesium ribbon turned into white powder and content were left to cool.

9. After wait of 10 minutes the content was crushed using stirring rod.

10. About 10 ml of distilled water was added in crushed powder and both were mixed with stirring rod.

11. The contents were again, with lid slightly ajar heated for 5 minutes (until the water evaporated).

12. After the water had evaporated the flow of gas was stopped and contents were allowed to cool. The observations about new compound were made.

13. Finally, the mass of cooled content was measured using balance.


Table 1:- Appearance of contents.

Content Characteristics

Magnesium ribbon 6.8 cm long


Silver colored


Magnesium oxide Dark colored

Small round balls like shape

Found in individual groups

White powder before water was added.

Table 2:- Mass of the contents

Content Mass

Crucible 27.175 g

Lid 12.221 g

Magnesium ribbon 0.124 g

Crucible + lid 39.396 g

Crucible + lid + Mg 39.520 g

Crucible + lid + MgO 39.602 g

Analysis and Calculations:

b) Some of the characteristic of chemical reaction includes appearing of new color, formation of bubbles, formation of precipitate and formation of new substance. As a result of our experiment we got a new substance, which is main evidence of chemical reaction. During heating of magnesium oxide the heat was observed by magnesium. During observation we noticed magnesium burning into white flame, which was supportive evidence as well. The changing of white powder into small-dark circles was a chemical reaction as well. Finally, it was difficult to reverse the reaction that...
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