Aleshia Otieno
Mr. Taylor
SCH3U-01
Lab #4
Monday, November 17, 2014
Introduction Percentage composition is the percentage of a formula mass represented by each element. Percentage composition compares the mass of one part of a substance to the mass of the whole. The law of definite proportions states that a specific compound always contains the same amount of elements in fixed proportions by mass. By finding the percent composition of magnesium oxide one will essentially be proving this law to be true.
Purpose The purpose of this lab is to determine the percent composition of magnesium oxide and determine whether the percentage is consistent.
Hypothesis Based …show more content…
on the law of definite proportions it is hypothesized that the percentage composition of magnesium oxide will be 60.3% magnesium, and 39.7% oxygen.
Hypothesis Calculations
Materials and Apparatus
8 cm strip of magnesium, Mg(s), ribbon
Small square of sandpaper or steel wool
Paper towel
Electronic Balance
Clean, dry crucible with lid
Bunsen burner
Retort stand
Ring clamp
Clay triangle
Welder’s goggles
Flint lighter
Crucible tongs
Ceramic pad
Procedure
1.
Observation tables were to be copied into lab books in order to record data.
2. Equipment was assembled as shown in diagram. (See figure 1.1 on page 2)
3. Welder’s goggles were used to protect the eyes when the magnesium was burning.
4. An 8cm long strip of magnesium was stripped of possible oxidation with a piece of sandpaper or steel wool. A piece of dry paper towel was used to remove any possible residue.
5. Based on the observation table the mass of the crucible with lid was recorded. A piece of magnesium was placed into the crucible with a lid and that mass was recorded as well.
6. With the lid off the crucible and the piece of magnesium strip inside, the crucible was placed onto the clay triangle. A Bunsen burner was lit to a high temperature (blue flame) with a flint lighter and placed directly under the crucible. Crucible tongs were then used to balance the lid on an angle allowing the crucible to mainly be closed while allowing little bit of oxygen into the crucible. Once the magnesium ignited the lid was placed on the crucible for a minute.
7. Crucible tongs were use to lift the side of the crucible lid in order to let in more oxygen.
8. Once the magnesium stopped igniting when exposed to oxygen Bunsen burner was turned off and crucible was left to …show more content…
cool.
9. A glass-stirring rod was used to make sure all magnesium reacted, and then the glass rod was cleaned using water.
10. The crucible was the placed back over flame until the water boiled off and magnesium oxide remained.
11. Crucible was removed from flame and allowed to cool for approximately 15 minutes. Mass of magnesium oxide was recorded.
12. Magnesium oxide was disposed of in a chemical waste bin.
Figure 1.1
Observations
Mass Observation Table (Table 1)
Mass of empty crucible and lid (g)
33.192g
Mass of crucible, lid and magnesium (g)
33.257g
Mass of crucible, lid and magnesium oxide (g)
33.304g
Mass of Magnesium Oxide produced (g)
0.112g
Mass of oxygen that must have reacted (g)
0.047g
Percent mass of magnesium in magnesium oxide
57.9%
Calculations/ Analysis/Discussion
Mass of magnesium used?
33.257g-33.192g= 0.065
Mass of magnesium oxide produced?
33.304g-33.192g= 0.112g
Calculate the mass of oxygen that reacted with the magnesium?
0.112g-0.065= 0.047
Calculate the percentage composition of magnesium oxide based on data above.
Mass of Mg= 0.065g
Mass of MgO= 0.112g
Percentage composition of Mg:
Because the percentage composition of magnesium is 57.9% we can take 100% and subtract 57.9% in order to get the percentage composition of oxygen, which is 41.2%.
Therefore the percentage composition of magnesium oxide based on the data from the lab is 57.9% magnesium, 41.2% oxygen.
Calculate the percentage error of Magnesium.
Therefore the percentage error is -3.98%
Suppose that some magnesium oxide smoke escaped during the investigation.
Would the Mg:O ratio have increased, decreased, or remained unchanged?
Mg will remain the same.
The amount of oxygen is determined by subtracting the mass of crucible, lid and magnesium from the mass of the crucible, lid and magnesium oxide.
33.304g-33.192g= 0.112g is the amount of oxygen produced. If some of the MgO was lost as smoke during the experiment and the amount of oxygen s less than 0.112g then the overall mass would be lighter because it would be oxygen leaving the crucible. Therefore the ratio will have more magnesium and less oxygen than it should have been. So one could say there has been an increase in the M:O ratio.
How would the value you calculated for the percentage composition of magnesium oxide have been affected if all the magnesium in the crucible had not reacted?
Using magnesium and oxygen’s mass on the periodic table let’s say only half of the Mg reacted you would have 12.15g of Mg that did not react with 12.15g+8g= 20.15g of MgO making the total mass that you weigh 32.30 (20.15+12.15). Which is less then the expected 40.3g for the mass of MgO.
The mass is going to be less due to the fact that no oxygen mixed with the unreacted
Mg.
Conclusion Based on the percentage composition of magnesium oxide on the periodic table 60.3% magnesium, and 39.7% oxygen and our percentage composition based on the data 57.9% magnesium, 41.2% oxygen this lab one could not prove the law of constant composition to be true. It is concluded that many errors took place in this experiment, they will all be explain in sources of error.
Sources of Error
It is predicted that there were many errors in this lab that caused the failure of the lab.
Magnesium oxide could have been lost during the heating process if smoke were to escape. Ensuring that the gap for oxygen to enter and leave the crucible wasn’t too large could have prevented this.
Magnesium could have not fully reacted with oxygen.
Magnesium oxide could have been lost when the glass-stirring rod was used to check if all the magnesium reacted. Pouring the water over the rod so that the Magnesium Oxide could fall back into the crucible.
Bibliography
Alfredo T. "Calculating the Percentage Composition of MgO| Socratic." Socratic.org. N.p., 9 July 2014. Web. 16 Nov. 2014.
"McGraw-Hill Ryerson EBook - Home." McGraw-Hill Ryerson EBook - Home. N.p., n.d. Web. 16 Nov. 2014.