Preparation of a Compound and Calculation
of an Empirical Formula
By Justin Wright
Portland Community College
General Chemistry II
2/6/2013
Abstract
The purpose of this experiment was to determine the empirical formula of magnesium
oxide. To produce the magnesium oxide, magnesium was reacted with oxygen in the air.
The initial mass of magnesium was then measured as well as the total mass of the
compound. These values were used to calculate the empirical formula for magnesium
oxide as well as the percent composition. A theoretical percent composition was also
calculated for the magnesium oxide. The theoretical percent composition of magnesium
oxide was determined to be 60.31% magnesium and 39.69% oxygen. The experimental
percent composition for the first trial of the experiment was 65.15% magnesium and
34.85% oxygen with respective percent errors of 8.025% and 12.19%. The results of the
second trial of the experiment were 60.31% magnesium and 39.69% oxygen with
respective percent errors of 12.58% and 19.12%. Both trials produced the experimental
empirical formula Mg1O1 for magnesium oxide.
Introduction
In the field of chemistry, the law of definite proportions states that compounds will
always contain two or more individual elements. These elements will always be
combined in a definite proportion by mass for a given compound (1). When working with
compounds, the empirical formula is a very important tool for many reasons. Not only do
empirical formulas list out all of the known elements of a compound, but they also
display the lowest whole number of moles and the relative number of atoms of each
element in a compound (2).
Since empirical formulas are a useful tool when working with compounds, it is important
to be able to determine the empirical formula for a compound when the formula is not
known. In this particular experiment, the empirical formula of magnesium oxide needed
to be determined through experimentation. Using a periodic table, the atomic weight of
magnesium was determined to be 24.31 grams and the atomic weight of oxygen was
16.00 grams (3). These values added together equal the molar mass of magnesium oxide.
Using these known values, the empirical formula of a compound such as magnesium
oxide can be determined by measuring the mass of each known component. The mass
measurements for each component are then used to calculate the total amount of moles
that exist for each of the elements. The ratio of magnesium oxide produced from the
amount of magnesium can then be determined and the empirical formula can be
constructed.
There are a few limitations to this experimental approach that could impact the
calculation of the empirical formula for magnesium oxide. If care is not taken when
heating the magnesium, some of it can be burned off as smoke. If this occurs, the final
mass of magnesium oxide will be lower than expected and could impact both the percent
composition by mass calculations and the empirical formula. One other limitation to this
experiment occurs when adding water to the ash. If any of the water remains after heating
the ash, it could result in a higher final mass of magnesium oxide. This would produce a
lower percent by mass calculation for the magnesium.
2 Experimental Methods
The first step of the experiment was to fire a crucible. The crucible was heated over a
Bunsen burner and then cooled to room temperature. The crucible was then weighed on a
scale and the mass was recorded in the lab notebook.
A sample of magnesium ribbon was then obtained from the lab cart and approximately
0.2 g was weighed out using a scale. The magnesium was polished excessively until the
ribbon did not have any noticeable dark spots. The magnesium was weighed again
following the polishing and the mass was recorded.
The magnesium was carefully put into the crucible using tongs and the lid was placed on
the crucible. The crucible was then placed on a clay triangle over a Bunsen burner. The
crucible was heated slowly at first until the entire bottom of the crucible glowed red. It
was then heated for approximately 15 more minutes. At this point the Bunsen burner was
turned off and the crucible was left in the clay triangle for several minutes to cool. It was
then removed from the triangle and placed on a mesh tile for further cooling.
Once the crucible was cooled, the lid was removed and the remaining ash was observed.
The crucible was then heated for a few more minutes to remove any remaining colors
from the ash. Three to five drops of water were then added to the ash and a small amount
of wafting was performed to check for any noticeable odors.
At this point in the experiment, the lid was then placed back on the crucible and it was
heated for another 5 minutes over the Bunsen burner. The crucible was allowed to cool to
room temperature and then placed on a scale to observe the mass. The final mass of
magnesium oxide was then determined by subtracting the initial weight of the empty
crucible from the final mass. The final magnesium oxide was then disposed in the
designated waste container.
In order to ensure that the first trial results were accurate, another lab group conducted a
second trial of this experiment. All of the steps listed above were performed during the
second trial and the data observed for each trial was swapped between lab groups.
Results
The experiment was completed and the magnesium successfully reacted with oxygen in
the air to form magnesium oxide. The experimental results and theoretical composition
percentages for each component in magnesium oxide are displayed in Table 1:
3 Table 1: Experimental Results
Trial 1
Substance
% Composition
(Experimental)
65.15
34.85
Mg
O
% Composition
(Theoretical)
60.31
39.69
% Error
% Composition
(Theoretical)
60.31
39.69
% Error
8.025
12.19
Experimental Empirical Formula: Mg1O1
Trial 2
Substance
% Composition
(Experimental)
67.90
32.10
Mg
O
12.58
19.12
Experimental Empirical Formula: Mg1O1
To calculate the values in Table 1, the molar mass of MgO was first determined using the
following calculation:
𝑀𝑜𝑙𝑎𝑟 𝑀𝑎𝑠𝑠 𝑜𝑓 𝑀𝑔𝑂 = 24.31 𝑔/𝑚𝑜𝑙 + 16.00 𝑔/𝑚𝑜𝑙 = 40.31 𝑔/𝑚𝑜𝑙
The molar mass was then used in the calculations below to find the theoretical percentage
by mass of each component in magnesium oxide:
% 𝑀𝑔 𝑏𝑦 𝑀𝑎𝑠𝑠 = 𝑂 𝑏𝑦 𝑀𝑎𝑠𝑠 = 24.31 𝑔 𝑀𝑔
1 𝑚𝑜𝑙 𝑀𝑔
1 𝑚𝑜𝑙 𝑀𝑔𝑂
× × × 100 = 60.31% 1 𝑚𝑜𝑙 𝑀𝑔
1 𝑚𝑜𝑙 𝑀𝑔𝑂
40.31 𝑔 𝑀𝑔𝑂
16.00 𝑔 𝑂
1 𝑚𝑜𝑙 𝑂
1 𝑚𝑜𝑙 𝑀𝑔𝑂
× × × 100 = 39.69% 1 𝑚𝑜𝑙 𝑂
1 𝑚𝑜𝑙 𝑀𝑔𝑂
40.31 𝑔 𝑀𝑔𝑂
Upon completion of both trials of the experiment, the recorded data from each trial was
used to determine the percent by mass of each component. The following equations were
used to calculate the percent by mass:
% 𝑀𝑔 𝑏𝑦 𝑀𝑎𝑠𝑠 𝑖𝑛 𝑀𝑔𝑂 = % 𝑂 𝑏𝑦 𝑀𝑎𝑠𝑠 𝑖𝑛 𝑀𝑔𝑂 = 𝑔 𝑀𝑔 (𝐼𝑛𝑖𝑡𝑖𝑎𝑙)
× 100
𝑔 𝑀𝑔𝑂 (𝐹𝑖𝑛𝑎𝑙)
𝑔 𝑀𝑔𝑂 (𝐹𝑖𝑛𝑎𝑙) – 𝑔 𝑀𝑔 (𝐼𝑛𝑖𝑡𝑎𝑙)
× 100
𝑔 𝑀𝑔𝑂 (𝐹𝑖𝑛𝑎𝑙)
The percent error for each percent by mass calculation was also determined using the
experimental data and the calculated theoretical percent by mass values for each
component. The percent error equation can be seen below:
4 𝑃𝑒𝑟𝑐𝑒𝑛𝑡 𝑒𝑟𝑟𝑜𝑟 = 𝐸𝑥𝑝𝑒𝑟𝑖𝑚𝑒𝑛𝑡𝑎𝑙 𝑣𝑎𝑙𝑢𝑒 – 𝑇ℎ𝑒𝑜𝑟𝑒𝑡𝑖𝑐𝑎𝑙 𝑣𝑎𝑙𝑢𝑒 × 100
𝑇ℎ𝑒𝑜𝑟𝑒𝑡𝑖𝑐𝑎𝑙 𝑣𝑎𝑙𝑢𝑒 The empirical formula for magnesium oxide was also calculated for each trial. In order to
calculate the formula, the mass values were first converted from grams to moles. The
calculations for the first trial can be seen below:
1 𝑚𝑜𝑙 𝑀𝑔
= 0.008482 𝑚𝑜𝑙 𝑀𝑔
24.31 𝑔 𝑀𝑔
1 𝑚𝑜𝑙 𝑀𝑔𝑂
𝑀𝑔𝑂 (𝑚𝑜𝑙𝑒𝑠) = 0.3165 𝑔 𝑀𝑔𝑂 ×
= 0.007852 𝑚𝑜𝑙 𝑀𝑔𝑂
40.31 𝑔 𝑀𝑔𝑂
𝑀𝑔 (𝑚𝑜𝑙𝑒𝑠) = 0.2062 𝑔 𝑀𝑔 ×
The calculated mole values were then divided by the smaller of the two results in order to
obtain the ratio of MgO produced from the amount of Mg. The calculation for trial 1 can
be seen below:
0.008482 𝑚𝑜𝑙 𝑀𝑔
= 1.080 ≈ 1
0.007852 𝑚𝑜𝑙 𝑀𝑔𝑂
In both experimental trials, it was observed that the ratio of Mg to MgO was one to one,
resulting in an empirical formula of Mg1O1 for magnesium oxide.
Discussion
There were a few limitations and possible sources of error during the experimental
process. Each of these items may have contributed to the high percent error values seen in
table 1 for both trials of the experiment.
During the first trial of the experiment, the crucible lid was lifted prematurely allowing a
small amount of smoke to escape. This may have resulted in a loss of mass of the final
sample. There was also a small amount of substance lodged in the tip of the pipet that
was used to break up the ash. An attempt was made to remove the substance but this
resulted in a small loss of sample. After the initial heating, a few pieces of magnesium
were visible in the ash and needed further cutting. The cutting process may have causes a
small amount of magnesium to be lost. Each of these items may have contributed to the
high percent error values for the first trial of the experiment.
During the second trial of the experiment, the crucible lid fell off while it was being
placed on the cooling pad. This resulted in a small amount of smoke escaping and may
have had an impact on the final mass observed. It would also explain the high percent
error values seen in the second trial of the experiment.
One experimental limitation is that magnesium can be burned off as smoke if it is not
carefully heated. Special care was taken to ensure that the magnesium was only heated
enough to remove any colors and chunks from the ash but it is possible that it was
overheated. The loss of magnesium as smoke would have impacted the final mass of
5 MgO. It would also have affected the calculations for percentage composition by mass as
well as the empirical formula.
The final limitation of this experiment occurs when the water is added to the ash. The
magnesium oxide was reheated after adding the water to ensure that any traces of the
water were removed. However, it is possible that some of the water was not removed
completely during the heating process. The remaining water would increase the mass of
the final magnesium oxide recorded. This would result in a lower percent by mass figure
for the magnesium and a higher percent by mass figure for the oxygen.
Conclusion
The theoretical percent composition of magnesium oxide was determined to be 60.31%
magnesium and 39.69% oxygen. The experimental percent composition for the first trial
of the experiment was 65.15% magnesium and 34.85% oxygen with respective percent
errors of 8.025% and 12.19%. The results of the second trial of the experiment were
60.31% magnesium and 39.69% oxygen with respective percent errors of 12.58% and
19.12%. Both trials produced the experimental empirical formula Mg1O1 for magnesium
oxide.
References
1. Experiment #1 Lab Handout: Preparation of a Compound & Calculation of an
Empirical Formula. Rock Creek Chemistry Department, United States of America.
2. Silberberg, Martin S. 2012. Chemistry: The Molecular Nature of Matter and
Change, 6th Edition. The McGraw-Hill Companies, United States of America.
3. Periodic.lanl.gov - Periodic Table of Elements. Los Alamos National Laboratory,
United States of America.
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