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Law of Constant Composition Lab Essay

Purpose: To determine the percent magnesium by mass in magnesium oxide and to observe if the percentage composition is constant by comparing class results.

Hypothesis/Prediction: The percent composition by mass of magnesium in magnesium oxide will not change significantly with each group that conducted the experiment. The composition of each substance should stay the same and any differences must be due to some error.

Materials:Magnesium stripCrucibleCrucible coverClay triangleIron ringRetort standTongsBalanceBunsen burnerProcedure:1.obtained a strip of magnesium between 30-40 cm long2.coiled magnesium strip into a tight roll3.measured the mass of the crucible and cover4.Added the magnesium strip to the crucible and measured the magnesium, crucible and cover together.

5.Partially covered the crucible with the cover and heated it using a Bunsen burner until the magnesium ignited.

6.Turned off Bunsen burner.

7.waited for combustion to proceed8.when the reaction appeared completed, heated the crucible again for another five minutes9.allowed crucible to cool for ten minutes10.measured the mass of the crucible, cover and magnesium oxideObservations:When ignited, the magnesium strip gave off a bright light.

There was a colour change on the magnesium.

It started out as silver and turned into a white colour.

The combustion gave off a distinct odour.

The end result was a white powder but some magnesium had been left in its original shape.

According to the theory of J.L. Proust, a compound always has the same percentage composition no matter how it is prepared. For all three of our own individual groups trials, we obtained the same percentage composition and this satisfies Prousts theory.

This is also accurate with what we have read in the textbook Chemistry 11 on the Law of Definite Proportions on pages 147 150.

2.What conclusion can you make upon the class results?The conclusions I can make based upon the class results are that the composition of a compound must vary because everyone had a different answer. In most cases, the percent composition of magnesium in the compound was very different from what my group achieved. However, sometimes, the classes results were quite similar to my own.

The theory that C.M. Berthollet introduced about the composition of a compound was that a compound has an infinite number of compositions depending on the proportions of the components that were used in its preparation. His theory satisfies the results obtained from the whole class. Since every group had a different amount of magnesium they must also have had a different composition of magnesium in the compound formed, magnesium oxide.

However, this is inconsistent with what we have been taught and with what is written in the textbook. Since the textbook is a more reliable source of information, I must conclude that the reason for this difference in results may be attributed to error on the part of the students conducting the experiment.

3.Which French Scientist would you tend to support? Explain.

The French scientist that I support is J.L. Proust. I agree with Prousts theory because I believe that if the composition of a substance were to change, so would its properties. An obvious example of this is one stated in the textbook: water and hydrogen peroxide. The simple addition of a hydrogen atom to the compound of water can make a liquid that is essential to life become deadly. Since I know that the percent composition of a particular substance is the same no matter where, how or when it is made, I must agree with J.L Proust and support his theory.

Sources of ErrorOne reason why a student may have obtained a higher percentage of magnesium than the rest of the class is: during the experiment, mass must have been lost. This could have been done in many ways. Often, the magnesium took too long to ignite and students began to put it straight into the flame to ignite it and then put it back into the crucible. Sometimes, the magnesium wilted and little pieces of it broke off. In addition, the magnesium would sometimes ignite and then extinguish itself. In the constant removal of the magnesium from the crucible, mass was lost in the form of ashes. When mass is lost during the experiment, the total mass of the compound is lower and when the mass of magnesium was divided by the total mass, yielded a higher percent of magnesium.

One reason why a student may have obtained a lower percentage of magnesium than the rest of the class is: the magnesium did not properly combust. In many cases, the magnesium would extinguish itself and would not burn completely. This resulted in some white powder, ash, and some magnesium still in the same shape that it was when we began the experiment. This suggests that it did not burn with the rest of the magnesium ribbon and therefore, did not form the compound with oxygen. This would yield in a higher total mass than what it should have been and when the mass of magnesium is divided by the total mass, would give a lower percentage.

The following calculation shows the actual percentage of Magnesium in the compound magnesium oxide.

Molar mass of MgO = 24.3 + 16.0= 40.3For 1.00 mol of MgO:%Mg = 24.3 / 40.3 x 100= 60.3%Therefore, the actual percentage value of Mg in MgO is 60%The following calculation shows the percentage error for my own groups result% error = (experimental accepted) / accepted x 100= (72 60) / 60 x 100= 20%Therefore the percentage error for my group was 20%.

ConclusionBased on the data collected by each group, the composition of a substance must change depending on the proportions of the components that were used in its preparation.

However, as explained earlier, the textbook and the properties of matter state otherwise. Scientifically, if the composition of a compound was to change, so would its properties. The results obtained by the class are quite the opposite of what has already been proven by Proust to be true and therefore, I believe that the evidence obtained by our chemistry class is the result of many mistakes and cannot be used to convey the idea of the Law of Constant Composition which states that the composition of a specific compound is constant.

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