EXPERIMENT
Elemental Mass Percent and
Empirical Formula from
Decomposition
1
OBJECTIVE
Prepared by Edward L. Brown, Lee University
The student will become familiar with laboratory apparatus/techniques useful
in combustions (crucible and lid) and decompositions. In addition, students
will manipulate masses obtained before and after the reaction to arrive at the
mass percent and empirical formula of the product (in synthesis reactions) or
reactant (in decomposition reactions).
A
APPARATUS
AND
CHEMICALS
P P A R A T U S
Ring Stand
15 cm Pyrex test tube
Bunsen Burner
Glass tubing (bent)
Rubber tubing
C
H E M I C A L S
Copper oxide (CuxOy)
An element’s mass percent is determined by weighing the various elements
comprising the compound and dividing those individual masses by the total mass of the
compound. Then, the compound’s empirical formula is determined by converting these
masses into moles (or atoms) and calculating the elemental molar ratio.
Elemental Mass Percent
Compounds containing stable elements can be assayed to determine their
elemental composition. A host of companies provide these elemental analysis reports to
academic and industrial clients. The techniques and instruments used in these companies
include Neutron Activation Analysis (NAA), Inductively Coupled Plasma – Mass
Spectroscopy (ICP-MS), and Proton Induced X-Ray Emission (PIXE), to name a few. At
the introductory level of elemental analysis, we will limit our focus to compounds
containing two (at most three) elements. In today’s lab, the synthesis of magnesium oxide
from magnesium metal provides the mass data needed to determine the mass percent of
both elements. Likewise in the decomposition reaction, you will begin by determining the
total mass of the copper oxide compound; the removal of oxygen from the compound will
provide the mass data needed to report the mass percent of copper and oxygen.
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Elemental Mass Percent and Empirical Formula
Empirical Formulas
The empirical formula of a compound is the simplest whole number ratio of its
elements. This ratio can be either an atomic ratio or a molar ratio. The experimental
determination of a compound’s empirical formula occurs in three steps:
Step 1:
Step 2:
Step 3:
Step 4:
Determine the mass of each element present
Convert the individual elemental masses into the number of moles of
each element.
Express the molar ratio of the two elements by dividing the number of
moles of each element by the smaller of the two numbers. This will
assign “1 mole” to the element with the smallest number of moles and
the other element will be assigned a number greater than or equal to “1
mole”.
This sequence of steps can be summarized using a table similar to
Table 1.1. Each element is assigned a column, so that we are not
limited by the ChemConneXions Map to just Elements A and B, but
can determine the Empirical Formula for compounds containing
multiple elements – our table will have multiple columns.
Sample Problem 1.1:
The electrolysis (decomposition) of water produced 23.2 mL of Hydrogen gas
(density = 0.08988 g/L) and 12 mL of Oxygen gas (density = 1.429 g/L). What is the
empirical formula of water?
Step 1: Mass of the Elements
0.002085 g H
0.01715 g O
Step 2: Moles of the Elements
2.085E-3 g ÷ 1.0079 g/mol
1.715E-2g ÷ 16 g/mol
2.0686E-3 mol H
1.072E-3 mol O
Step 3: Divide by smallest moles
Step 4: Write the empirical formula
1.93 mol H
1.00 mol O
H2O1 or simply H2O
Percent Composition from Empirical Formulas
The percent composition of a compound is easily calculated if its empirical
formula is known. An empirical formula gives us two essential facts about a compound’s
elemental composition: the identity of the elements and their relative ratios. For example,
water (H2O) is composed of 2 atoms of hydrogen and 1 atom of oxygen. This
information, coupled with information displayed in the Periodic Table, allows the
determination of the percent hydrogen and oxygen in water.
Experiment 1
1-2
Elemental Mass Percent and Empirical Formula
Sample Problem 1.2:
Determine the percent hydrogen and oxygen in water.
Step 1: Moles of the Elements
Step 2: Mass of the Elements
2 mol H
1 mol O
2 mol H×1.008g/mol 1 mol O ×16 g/mol
2.016 g H
16 g O
Step 3: Add the masses together to find the total mass
18.016 g H2O
Step 4: Divide the elemental masses by the total masses and multiply by 100
(2.016 / 18.016)×100
11.19 % H
(16 / 18.016)×100
88.81 % O
Obviously, the reversal of these four steps will give the empirical formula from the percent
composition. It should be noted that the mass percent of hydrogen and oxygen in water
will always be 11.19 % and 88.81 % regardless of the source of the water. This
observation led to the LAW OF DEFINITE PROPORTIONS.
Molecular Formulas from Empirical Formulas
While the empirical formula by definition shows the smallest whole number ratio
of the elements, it may or may not be the molecular formula. The molecular formula
reveals the actual elemental composition in a distinct molecule. For example, the
molecular formula of hydrogen peroxide is H2O2 but the smallest whole number ratio of
the elements is HO (its empirical formula). We can obtain the molecular formula from the
empirical formula if the molecular weight of the compound is known. It turns out that the
molecular formula is the same whole number multiple of the empirical formula as the
molecular mass is of the empirical mass. Comparing H2O2 (molecular formula) to HO
(empirical formula) shows that the whole number multiple is “2”. Two times HO will
give H2O2. In addition, the empirical mass of HO is 17.007 g/mol (1.0079 + 15.999)
while the molecular mass of H2O2 is 34.014 g/mol ((1.0079×2)+(15.999×2)) – again a
multiple of “2”.
A template has been developed to guide you through the calculations in a problem
of this type. This template is a combination of the approaches used previously in Sample
Problem 1.1 and 1.2. After reading a problem, place the starting information into this
template and fill in all missing information between your starting point and the answer.
Sample Problem 1.3:
The organic compound n-octylacetate belongs to a class of generally pleasant smelling
compounds called esters. In fact, n-octylacetate smells like oranges. If this ester contains
Experiment 1
1-3
Elemental Mass Percent and Empirical Formula
69.72 % C, 11.70 % H, and 18.58 % O by mass, what is its molecular formula? The molar
mass of n-octylacetate is 172.3 g/mol.
Answer: The starting information is entered into the Empirical / Molecular Formula
Template in bold. Since the question asks for the molecular formula, all information
between the % composition and molecular formula is required.
Empirical and Molecular Formula Template
Fill in the starting information, the answer and everything between the two. All other
fields must be left blank.
Element Symbol
C
H
O
Percent (%)
69.72
11.70
18.58
11.70
18.58
1.0079
15.999
Mass (g)
69.72
(if % is given, assume 100 g)
Elemental Molar Mass (g/mole)
12.011
(report to 1 SF > than starting number)
Moles
5.805
11.61
1.161
Empirical Molar (or Atomic) Ratio
(whole numbers only)
5
10
1
Empirical Formula
C5H10O
Empirical Mass (g/mole)
86.13
Molecular Mass (g/mole)
172.3
Molecular Molar Ratio
10
Molecular Formula
20
2
C10H20O2
Table 1.1
Decomposition of CuxOy
Copper oxide exists as either Copper I (cuprous) or Copper II (cupric) oxide. Heating
either of these compounds with methane (CH4) gas will produce copper metal. Both of
these compounds have valuable chemical applications: cuprous oxide is used as a
Experiment 1
1-4
Elemental Mass Percent and Empirical Formula
fungicide and an insecticide primarily in the fruit industry, cupric oxide has found
notoriety in the area of superconductivity when it appears as a blend of yttrium, barium
and copper oxides.
1. Flame dry a Pyrex test tube (15 cm or longer) by holding it vertically (mouth upward) PROCEDURE
with tongs in the flame of a Bunsen burner for 3 – 4 minutes. Rest the hot test tube on
a clean dry surface until it has cooled to room temperature (about 5 minutes).
Determine the mass of the test tube [Data Sheet Q1] by taring a small beaker on the
balance and then placing your test tube in the beaker.
2. After recording the mass of the test tube, tare the scale with the test tube / beaker on it.
Remove the test tube from the beaker and add 1.000 g ± 0.200 g of Copper oxide.
Rubber tubing
to gas outlet
9 hold the test tube over the copper
oxide container and add a little
copper oxide
9 gently place the test tube in the
beaker and note the mass
9 if the mass is 1.000 g ± 0.200 g,
lower the draft shield and record the
exact mass [Data Sheet Q2], if not,
add more copper oxide
9 Record the 4 digit Unknown
Number of the Copper Oxide you
used [Data Sheet Q3].
3. Clamp the test tube to a ring stand with a
bare metal test tube clamp.
4. Assemble the remaining apparatus as shown
in Figure 1.1 but DO NOT TURN ON THE
GAS.
5. Next, light only the Bunsen burner – THIS
6.
Figure 1.1
SHOULD NOT BE AN EXTREMELY HOT
FLAME.
HAVE YOUR INSTRUCTOR OR TA ASSIST YOU WITH THE LIGHTING OF THE GAS
THAT EXITS THE TEST TUBE.
•
Position the ring stand so that the test tube is well away from your face and
arms.
• Hold the Bunsen burner in your right hand and reach behind the ring stand
with your left hand to turn on the Gas.
• Move your lit Bunsen burner under the mouth of the test tube and SLOWLY
turn on the gas to the test tube.
• Adjust the gas flow to the test tube such that the flame is only 1 – 1.5 inches
high.
7. Heat the Copper oxide with a cool flame for 5 minutes.
8. Then, adjust the Bunsen burner to produce a hot flame. Heat the test tube with this hot
flame for 10 – 40 minutes (depending on the type of copper oxide used). You will see
a copper colored compound form during this heating process.
9. When the entire solid is a copper color, sweep any condensed water out of the test
tube by moving the Bunsen burner along the top and bottom of the test tube.
Experiment 1
1-5
Elemental Mass Percent and Empirical Formula
10. Turn off the gas to the Bunsen burner, but allow the flame to keep burning at the
mouth of the test tube while the rest of the test tube cools down.
11. After 5 minutes, turn off the gas to the test tube and remove the glass tubing
(Caution: HOT!). Allow the test tube to cool to room temperature.
12. Record the mass of the test tube and Copper metal [Data Sheet Q4].
13. Remove the Copper from the test tube and rub it gently on a hard surface (i.e. the
slate lab counter top) to produce the familiar shiny look and color of copper. You may
dispose of this copper metal in the trash OR keep it as a lab souvenir.
14. Determine the mass of the Oxygen contained in the Copper oxide [Online Report
Sheet Q5].
15. Determine the mass percent of Copper [Online Report Sheet Q6] and Oxygen
[Online Report Sheet Q7] in the Copper oxide.
16. Determine the moles of Copper [Online Report Sheet Q8] and Oxygen [Online
Report Sheet Q9] in the Copper oxide.
17. Determine the molar ratio (don’t round the answer) between Copper [Online
Report Sheet Q10] and Oxygen [Online Report Sheet Q11] in the Copper oxide.
18. Give an empirical formula for Copper oxide (Round To Whole Numbers – One
Number Will Be A “1”) [Online Report Sheet Q12].
19. Write a balanced chemical equation for this reaction assuming you started with
Copper (I) oxide [Online Report Sheet Q13].
20. Write a balanced chemical equation for this reaction assuming you started with
Copper (II) oxide [Online Report Sheet Q14].
Waste Disposal
The elemental copper can be placed in the regular trash container.
Lab Report:
Once you have turned in your Instructor Data Sheet, lab attendance will
be entered and lab attendees will be permitted to access the online data / calculation
submission part of the lab report (click on Lab 1 – Empirical Formula Copper Oxide).
Enter your data accurately to avoid penalty. The lab program will take you in order to
each calculation. If there is an error, you will be given additional submissions (the number
and penalty to be determined by your instructor) to correct your calculation.
Post-Lab Questions:
The questions for this lab can be found at
http://www.Chem21Labs.com. Do Not Wait Until The Last Minute!!!! Computer
Problems and Internet Unavailability Happen, But Deadlines Will Not Be Extended!!
On the Internet, complete the Post Lab Assignments for Laboratory 1. The computer
program will check your answer to see if it is correct. If there is an error, you will be
given additional submissions (the number and penalty to be determined by your instructor)
to correct your answer.
Late Submission: Late submission of the lab data / calculations is permitted with
the following penalties: - 10 points for submissions up to 1 day late, - 20 points for
submissions up to 2 days late.
Experiment 1
1-6
Laboratory 1
Student Data Sheet
1. Mass of the test tube
g
2. Mass of the copper oxide
g
3. Unknown Number of the Copper Oxide (4 digits)
4. Mass of the test tube and copper metal
g
Name:
Laboratory 1
Instructor Data Sheet
1. Mass of the test tube
g
2. Mass of the copper oxide
g
3. Unknown Number of the Copper Oxide (4 digits)
4. Mass of the test tube and copper metal
Experiment 1
g
1-7