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Percentage composition by mass of Magnesium oxide

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  • Pages: 7
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  • Category: Chemistry

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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 occurred.

c) Mass of crucible 27.175 g

Mass of lid 12.221 g

Mass of magnesium ribbon 0.124 g

Mass of Mg + lid + crucible 39.520 g

Mass of MgO + lid + crucible 39.602 g

Mass of O =?

Mass of O = (Mass of MgO + lid + crucible) – (Mass of Mg + lid + crucible)

= (39.602 g) – (39.520 g)

= 0.082 g

The mass of oxygen that reacted with the magnesium is 0.124 grams.

d) mMgO = 0.206

mMg = 0.124

mO = 0.082

% Mg = (mMg / mMgO) x 100

= (0.124 g / 0.206 g) x 100

= 60.194

% O = (mO / mMgO) x 100

= (0.082 g / 0.206 g) x 100

= 39.806

e) As stated in the law of definite proportion the percentage composition of Magnesium oxide was approximately same for our classmates. The following table shows the results of two other groups.

Table 3: Percentage composition of MgO for classmates

Name of group or procedure Percentage composition for Mg Percentage composition for O Relatively related to the required?

Molar mass value from periodic table 60.31% 39.69 % Required

Pragnesh, Gopal and Soiendeep 60.19% 39.81% Yes

Bhavik, Gagandeep, and Jasdeep 61.63% 38.37% Yes, but higher.

Aditi, Manmeet and Kamaljit 60.23% 39.77% Yes

Samina, Sweta and Rutva 58.31% 41.69% Yes, but low

From this table it can be proved that the percentage composition for magnesium and oxygen is relatively similar for every single Magnesium oxide reactions. My classmates had difference of 2%-5% in their results.


Based on my and other classmates’ observations and results it can be proved that every time magnesium combines with oxygen to form Magnesium oxide the percent composition for magnesium will be 60.31% and for oxygen it will be 30.69 % (the results achieved by experiments are not accurate). The definite proportion of magnesium and oxygen supports the law of definite proportion that states that the pure substance contains the same portion of elements every time they combine in same manner.


f) If some of the magnesium oxide has escaped from the crucible the percentage composition of magnesium will neither go up nor down. The percentage composition of magnesium will stay same even though some of the magnesium oxide has lost. First of all, we just proved that no matter what the mass is, every time oxygen combines with magnesium to form Magnesium oxide, the percentage composition of both elements will stay same. Other way, if some of the Magnesium oxide has escaped it would have been escaped in 60.31%: 39.69% (ratio of 3:2, Mg: O).

g) If magnesium had reacted with some other component in the air, the percentage composition of magnesium’s sharing with oxygen will be lower than in MgO. In this situation the magnesium that will combine with oxygen will be either in form of Magnesium carbide, Magnesium nitride or Magnesium hydride. This will divide magnesium’s sharing in two parts, one with oxygen and second with other particles of air. As a result of this totally new substance will form, which will have magnesium’s lower sharing with oxygen.

On other hand, since the mass of air particles like nitrogen, carbon and hydrogen is lower than oxygen, the percentage composition of magnesium will be higher in Magnesium nitride, Magnesium carbide, or Magnesium hydride than it is in Magnesium oxide.

h) It was necessary to polish magnesium ribbon before using in order to remove the unnecessary materials from it that might affect the experiment. First of all, corroded part of magnesium ribbon might contain some of the oxygen, meaning that the weight of magnesium and oxygen would have been inaccurate from beginning. More over while magnesium was kept in laboratory or open place it would have reacted with other particles of air or some chemicals in laboratories. This, again, will

impact the result of investigation. That’s why in order to get more accurate results it was necessary to polish magnesium ribbon before using it. Polishing might also make it easier for magnesium to react with oxygen.

i) In this investigation the main problem we faced is that the reaction of magnesium with other particles of air. In order to solve this problem the experiment would have done in glass box with two openings. Here we can make oxygen flow from one opening and other as exit. Since the air will be traveling from warmer to cooler areas no other particles then air will be entering, which will provide us with accurate results. So basically, we need a set up that will prevent magnesium’s reaction with other particles of air.

j) Based on the evidence obtained from classmates, as I predicted the law of definite proportions is valid. This is because the results of other groups were not more than 5% different than ours (Table 3). Since the results of the students were relatively same to the result obtained from the periodic table’s values, it can be proved that every chemical compound contains definite and constant proportion (by weight) of its constituent elements.

Sources of Experimental Error:

Since no science activity is perfectly done there were few errors that occurred during our investigation. First of all, we had to start off with a smaller burner and we had our reaction going slower than other groups. This caused us to run out of time for further steps of experiment. As a result we didn’t give much time for last content to cool down, which is mainly the reason of our not exact results.

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