Percentage Composition of Magnesium Oxide

Percentage Composition of Magnesium Oxide Purpose: This lab was conducted to find the percentage composition of magnesium oxide and to see whether enough magnesium nitride forms to noticeably alter the percentage composition of magnesium oxide, testing the law of definite proportions. To do this, you take the magnesium strip and place it into the crucible to burn it. After weighing and recording, add distilled water to the crucible and put it back up to burn with the product. You then check the weight of that product and compare it with the other product. Introduction: The law of definite proportions states that “the elements in a compound are always present in the same proportion by mass. So regardless of the size of the sample, the percent composition should always have the same percent composition just as compound has the same chemical formula”. (Mr. Hunt 11/09/17). The percent composition is important because it “tells what percentage each element makes up in the compound. If samples of compounds are unknown, combustion analysis of the unknown substance can be carried out to determine the percent composition of the compound”. (Mr. Hunt 11/09/17). A hypothesis was made for this lab stating that, “the formation of magnesium nitride will affect the percent composition of magnesium oxide in this experiment.” (Landon Jeffrey 11/21/17). The reasoning behind this hypothesis is that “there is more nitrogen in the air than oxygen. There is 78% nitrogen and 21% oxygen in the air. Almost four times as much nitrogen than oxygen”. (Landon Jeffrey 11/21/17). Experimental: The first thing you do is record the mass of the empty crucible and lid. Then, after polishing with steel wool, put the magnesium strip inside the crucible. Then place the crucible on the clay triangle resting on the ring clamp and light the Bunsen burner below. Be sure that the crucible lid leaves an opening so oxygen can enter the crucible. Then heat the magnesium until it no longer flares. Remove the crucible from the stand, with crucible tongs, and measure the mass of the product, then proceed. After crushing the magnesium with a glass stirring rod, add 5 mL of distilled water to the crucible and heat it once more. Then finish by measuring the contents of the crucible. Results: Table 1 Observations Before, During, and After Part A Observations Before Thin magnesium strip During Flaring, red strip After Ashy product Part B Observations Before Ashy product in water During Ash with some water evaporated After Ashy product with all water evaporated Table 2 The Mass Before The Experiment Magnesium with Crucible and Lid 51.82g Crucible with Lid 51.32g Magnesium 0.49g Table 3 The Mass After The Experiment Product Contents after burning 0.63g 52.02g Table 4 The Mass After Part B Product B Contents after burning 0.70g 52.02 Calculations/Discussion: Percent composition can be calculated using the formula: %element=m element/m sample x 100% You would use this equation on every element in a compound to find its total percent composition. The percent composition for MgO goes as follows: m Mg=24.31g %Mg=24.31g/40.31g x 100%=60.31% m O=16.00g %O=16.00g/40.31g x 100%=39.69% m sample=40.31g therefore the percent comp of O is 39.69% and Mg is 60.31 To find the percent composition of the experiment done in part A you must first find the mass of each individual element. Since you are already given the mass of magnesium, through weighing it before experimentation, all you need to do is take the product and subtract the mass of the magnesium and the remainder will be oxygen. In this case it would be 0.63g - 0.49g = 0.14g. Using this info you can proceed with the formula below: m Mg=0.49g %Mg=0.49g/0.63g x 100%=78% m O=0.14g %O=0.14g/0.63g x 100%=22% m sample=0.63g therefore the percent comp of O is 22% and Mg is 78% However you must keep in mind that some of that oxygen is nitrogen as well. This is because nitrogen also makes up the air, so when the oxygen was burned into the magnesium, nitrogen was burned in as well. Therefore there is now magnesium oxide as well as magnesium nitride. To change the magnesium nitride into magnesium oxide you add distilled water, which reacts with magnesium nitride to form magnesium hydroxide. When magnesium hydroxide is heated it becomes magnesium oxide. Using the logic from part A’s percent composition equations, we can easily find the percent composition of oxygen. We do this by subtracting the original magnesium’s mass from the sample. In this case it would be 0.70g - 0.49g = 0.21g. Using this info you can proceed with the formula below: m Mg=0.49g %Mg=0.49g/0.70g x 100%=70% m O=0.21g %O=0.21g/0.70g x 100%=30% m sample=0.70g therefore the percent comp of O is 30% and Mg is 70% The adding of water changed the percent compositions of both by 8%. I think this is because the oxygen was more abundant from there being no nitrogen in the product. In the standard formula for MgO the percent composition was 39.69% for O and 60.31% for Mg. When finding percent composition for part A the percent composition was 22% for O and 78% for Mg. When finding percent composition for part B the percent composition was 30% for O and 70% for Mg. When comparing all these percentages together you see that they aren’t very far apart from each other, in fact, the largest gap is a difference of 17.69%. A trend that has stayed the same, however, is that the percent composition of oxygen has always stayed lower than that of the percent composition of magnesium. The lab did not experience any errors that would affect the results made. Conclusion: So in the end, there were two findings of the percent composition of magnesium oxide, part A’s being 22% O and 78% Mg, and part B’s being 30% O and 70% Mg. These two pairs of percentages did not quite match up, nor were they too different than the theoretical percent composition. This being the percent composition of 39.69% O and 60.31% Mg. The differences between the theoretical percent composition and part A’s percent composition was the largest difference, with a difference of 17.69%. In comparison, the differences between the theoretical percent composition and part B’s percent composition was far smaller, being a difference of only 9.69%. From these numbers you could state that the law of definite proportions does work due to the fact that the percentages were closer together after the nitrogen was removed from the product. This conclusion can also be drawn by looking at how close the numbers are to each other.