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05 06 report

Percent Yield Lab Report
Instructions: For this investigative phenomenon, you will need to determine the percent yield of
magnesium oxide from the given reaction to determine if it is a useful commercial process.
Record your data and calculations in the lab report below. You will submit your completed
Title: Testing the Percent Yield of Magnesium Oxide
Objective(s): Observe a reaction between the magnesium and the oxygen
No hypothesis needed for this lab. Your theoretical yield calculation serves as your prediction for
what you expect the lab to produce, and that will be determined later in the lab.
1.Select and weigh a crucible to find the mass of it
2.Record the mass of the empty crucible
3.Cut a piece of metal from the ribbon
4. Remove the oxidized magnesium
5. Put the magnesium strip in the crucible
6.Weigh the mass of the crucible, lid and magnesium
7. Place the crucible on the burner for 10 minutes.
8. Turn off the burner and wait 5 minutes.
9. Weigh the crucible again and record the data.
-Metal Strips
-Bunsen Burner
-Magnesium Ribbon
Remember, controlled variables are factors that remain the same throughout the
experiment. An independent (test) variable changes so that the experimenter can see the
effect on other variables. The dependent (outcome) variable will change in response to
the test variable.
Controlled variables: Mass of the empty crucible, length and mass of metal
Independent Variable: Time spent on burning and cooling the metal and oxidized
Dependent Variable: Mass of the oxidized metal, crucible, and lid
Summary of Steps:
Begin by weighing the crucible by itself, then with a steel wool, rub a specified length of
metal (ex: 1 in) until it is oxidized and cut it into a ball of metal strips. Then place it on
top of a Bunsen burner for approximately 10 minutes and then cool for 5 minutes. Repeat
the same experiment with a different cut of metal (ex: 2 in).
Type the data in the data table below. Don’t forget to record measurements with the correct
number of significant figures. Hint: Using the same instrument, you should have the same
number of digits to the right of the decimal.
Mass of empty crucible with lid
Mass of Mg metal, crucible, and
Mass of MgO, crucible, and lid
Trial 1
Trial 2
26.697 g
26.948 g
26.698 g
27.011 g
27.109 g
27.209 g
Show your calculations for each of the following. Remember, calculations should
follow rules for significant figures.
1. Write the balanced chemical equation for the reaction you are performing.
2Mg(s) + O2(g) (arrow) 2MgO(s)
2. Subtract the mass of the crucible and lid (row 1 in the chart) from the total mass
of Mg, crucible, and lid (row 2 in the chart) to find the mass of magnesium for
each trial.
 Trial 1: 26.948 – 26.697 = 0.251 g
 Trial 2: 27.011 – 26.698 = 0.313 g
3. Subtract the mass of the crucible and lid (row 1 in the chart) from the total mass
of MgO, crucible, and lid (row 3 in the chart) to find the mass of magnesium
oxide for each trial. This is the actual yield of magnesium oxide for each trial.
 Trial 1: 27.109 – 26.697 = 0.412 g
 Trial 2: 27.209 – 26.698 = 0.511 g
4. Magnesium is the limiting reactant in this experiment. Calculate the theoretical
yield of MgO for each trial.
Molar Masses:
Mg = 24.305 g/mol
X = Mg
O2 = 15.999 g/mol
MgO = 40.304 g/mol
Y = MgO
Given X g [molar mass X ] [ mole ratio from balanced reaction] [molar mass Y]
Given X g [1 mol x/ g X] [ mol Y/mol X] [ g Y/1 mol Y] = g Y
Trial 1: 0.251 g (1mol / 24.305 g/mol) (2 mol / 2 mol) (40.304 g/mol / 1 mol) = 0.416 g
Trial 2: 0.313 g (1mol / 24.305 g/mol) (2 mol / 2 mol) (40.304 g/mol / 1 mol) = 0.519 g
5. Determine the percent yield of MgO for your experiment for each trial.
Percent Yield = Actual / Theoretical
Trial 1: 0.412 g / 0.416 g = 0.99 x 100 = 99%
Trial 2: 0.511 g / 0.519 g = 0.986 x 100 = 98.6%
6. Determine the average percent yield of MgO for the two trials.
0.99 + 0.986 = 1.976 / 2 = 0.988 x 100 = 98.8%
Write a conclusion statement that addresses the following questions:
A logical reason when discussing why the product was larger than the reactant was due to the
burning of the Magnesium. While it burned, it lost some mass and decreased in size. Percent
yield can never exceed 100%. How long the magnesium burned could have affected it. The
experiment can be further explored by using a different element other than magnesium or
changing freezing it instead of burning it. The product had a higher mass than the reactant
due to the conservation of matter. The conservation of matter states that matter cannot be
created nor destroyed but transferred. In this case the reactants were transferred into a larger
Post-Lab Reflection Questions
Answer the reflection questions using what you have learned from the lesson and your
experimental data. It will be helpful to refer to your chemistry journal notes. Answer questions in
complete sentences.
1) When conducting this experiment, some procedures call for heating the substance several
times and recording the mass after each heating, continuing until the mass values are constant.
Explain the purpose of this process and how it might reduce errors.
The purpose of this process is to level out the substances, so they are constant. Not to
forget, we record the data to show the transition of two separate masses into the same.
When doing this we prevent errors by allowing them to be the same mass, we will likely
not run into any discrepancies due to their correlation. An error from the experiment that
I observed was weighing the metal after it was brushed. This affected the mass of each
substance and whether or not it would stay constant.
The reasoning for this process is to assess how each substance could show variability. We
do this to check for the consistency over a specific period of time.
2) Your company currently uses a process with a similar cost of materials that has an average
percent yield of 91 percent. If the average percent yield of this process is higher than that, this
could save the company money. What is your recommendation to the company? Please support
your recommendation using your data, calculations, and understanding of stoichiometry gathered
from this lab.
According to my data, my average yield was 98.8%. In order to increase their percent yield and
save money. Instead of burning the magnesium or scraping it. Leave it be. According to my data,
my numbers have showed that leaving the element be will allow for more mass and a larger
average percent yield.
To further explain, they should be using my process. My process clearly shows a high percentage
yield and gives the company more room to spend their money on other necessities.