LAB 14
DETERMINATION OF AN EMPIRICAL FORMULA
Grade Level Indicators:
 Show how atoms may be bonded together by losing, gaining or sharing electrons and that in a chemical reaction, the
number, type of atoms and total mass must be the same before and after the reaction (e.g., writing correct chemical
formulas and writing balanced chemical equations).
 Research and apply appropriate safety precautions when designing and conducting scientific investigations (e.g., OSHA,
Material Safety Data Sheets [MSDS], eyewash, goggles and ventilation).
 Develop oral and written presentations using clear language, accurate data, appropriate graphs, tables, maps and
available technology.
 Draw logical conclusions based on scientific knowledge and evidence from investigations.
Introduction:
In this experiment, you will determine the empirical formula of a compound. In so doing you will gain a clear
understanding of the difference between an empirical formula and a molecular formula.
An empirical formula gives the simplest whole number ratio of the different atoms in a compound. The empirical
formula does not necessarily indicate the exact number of atoms in a single molecule. This information is given by the
molecular formula. For certain compounds, the empirical formula and the molecular formula are the same; for other
compounds the empirical and molecular formulas are different. In all cases, the molecular formula is a simple multiple
of the empirical formula. Consider the following examples.
Experiments have shown that any sample of pure water contains two atoms of hydrogen for every atom of oxygen. The
empirical formula for water is, therefore, H2O. Various molecular formulas; H2O, H4O2, H6O3, and so on, are possible.
Each of these formulas expresses the same ratio of hydrogen and oxygen atoms as is expressed in the empirical
formula. Scientists have shown, however, that each water molecule actually consists of two atoms of hydrogen bound
to one atom of oxygen. Therefore, the molecular formula for this compound is H2O. For water, the empirical and
molecular formulas are identical.
For other compounds, the empirical and molecular formulas are different. Consider hydrogen peroxide. This
compound contains one atom of hydrogen for each atom of oxygen, and its empirical formula is HO. There is,
however, no stable molecule having this formula. In fact, it has been shown that individual hydrogen peroxide
molecules contain two atoms of hydrogen and two atoms of oxygen. The molecular formula of this compound is,
therefore, H2O2.
In some cases, two or more different compounds share the same empirical formula. This is true of acetylene and
benzene. Each of these compounds has the empirical formula CH. The molecular formula of acetylene, however, is
C2H2, while that of benzene is C6H6.
In this experiment, you will determine the empirical formula of magnesium oxide, a compound that is formed when
magnesium metal reacts with oxygen gas. In determining this empirical formula, you will make use of the law of
conservation of mass. According to this law, the total mass of the products of a chemical reaction must equal the total
mass of the reactants. (mass of Mg + mass of O2 = mass of MgxOy)
Therefore, knowing the mass of magnesium used and the mass of magnesium oxide produced in this reaction,
you can determine the mass of oxygen used. This ratio between the number of moles of magnesium used and the
number of moles of oxygen consumed can then be calculated and the empirical formula of magnesium oxide can be
written on the basis of this ratio.
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PRELAB (All answers on your own notebook paper!)
1. Butane in a lighter has a molecular formula, C4H8, what is the empirical formula for this compound?
2. Why must you heat the crucible and lid to dry both? (Why is not drying the crucible and lid with a paper
towel not sufficient?)
3. There are two chemical reactions that will occur when heating the magnesium metal on Day 2 of the lab, write
a balanced chemical equation for each including states of matter. (Hint: What are the two most abundant gases
in the atmosphere that magnesium could react with and what category of reaction are both reactions?!?)
4. Why do you add water on Day 3? Write a balanced chemical equation for this reaction. (Hint: The product
not really wanted from your previous balanced chemical equations will be the reactant and the products will be
one that is wanted and the other has a recognizable odor.)
5. Why must you be careful in looking at the burning magnesium ribbon inside the crucible?
6. How many TOTAL mass readings will be taken in this lab?
7. How do you test to see when the crucible is cool enough to measure its mass?
8. In the laboratory, a clean, dry crucible and lid was massed to be 30.021g. Then a sample of pure nickel was
placed into the same clean, dry and weighed crucible and lid and massed at 31.072 g. The crucible was heated
so that the nickel could react with oxygen in the air. After the reaction appeared complete; the crucible was
allowed to cool and the mass was determined to be 31.945 g. Determine the empirical formula of the nickel
oxide. Not sure how to do this problem. Look at the 7 steps for the calculations for this lab
and follow them for this problem. Just remember in the lab we are using magnesium metal
and in this question the metal is nickel.
MATERIALS:
Safety goggles, gas burner, electronic balance, clay triangle, crucible (with lid), ring support, ring stand, crucible tongs,
magnesium ribbon, ceramic plate
SAFETY:
Handle the crucible only with the tongs. There is a significant burn hazard associated with the handling of crucibles
because a hot crucible looks exactly like a cold crucible.
Remove the gas burner from beneath the crucible before using the crucible tongs to remove the crucible and its lid.
Use the tongs to grasp the lid by its porcelain knob; the crucible should be grasped by its edge.
PROCEDURE:
DAY 1
1. Place the crucible on a clay triangle balanced on a ring support clamped to a ring stand. Light the gas burner
and adjust it to give a clear blue flame (A yellow flame will deposit soot on the crucible and cause a large error
in your data.). Place the burner under the crucible. Adjust the height of the ring support so that the bottom of
the crucible is in the hottest part of the flame. Place the crucible lid slightly ajar on the crucible. The crucible
lid should be large enough to fit loosely down over the crucible edge.
2. Heat the crucible so that its bottom glows red for five minutes. Remove the burner and allow the crucible and
crucible lid to cool. This will take at least 5 minutes. CAUTION: The crucible gets extremely hot. Always use
crucible tongs in handling this piece of equipment. Use crucible tongs to transfer the crucible and lid to a
ceramic plate. When the crucible and lid are cool (when back of the hand brought near the crucible does
indicate significant heat being emitted), use crucible tongs to transfer them to a balance. Do not place a hot
crucible on the balance. Inaccurate mass readings and damage to the balance may result. Determine the mass
of the empty crucible and lid. You may want to do each separate in case you break the lid in later lab
procedures. Record this mass in the DATA TABLE.
3. Cut and then loosely coil a 25-cm length of magnesium ribbon and place it in the bottom of the crucible.
Determine the combined mass of the crucible, lid, and the magnesium. Record this mass in the DATA
TABLE.
4. Place crucible with magnesium inside and covered by the lid in the designated tray in back of room!
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DAY 2
5. Place the crucible, without its lid, on the clay triangle. Heat the crucible strongly until the magnesium ignites.
When the magnesium begins to burn, immediately place the cover on the crucible (using tongs) and remove
the burner. CAUTION: Do not look directly at the burning magnesium. The intense light may hurt your eyes.
Be careful to keep the crucible at arm's length at all times. Do not inhale the "smoke" produced.
6. After the reaction has subsided and "smoke" production has ceased, replace the burner and continue to heat the
crucible. Every 1 to 2 minutes, remove the burner and check the progress of the reaction by using tongs to lift
the lid of the crucible.
CAUTION: Do not lean over the crucible.
7. Then replace the lid and again apply heat. After 3 minutes of heating, remove the burner and check the
product. When the reaction is completed, the magnesium should be completely converted to a light gray
powder, magnesium oxide.
8. Place crucible (with magnesium oxide product inside) with lid in designated tray in back of room!
9. As a precaution (in case crucible is loss or breaks) mass the crucible with product. Record in the DATA
TABLE.
DAY 3
10. Add 10-12 drops of distilled water to the product in the crucible. As the drops are being added check for any
odor by wafting fumes towards you nose.
11. Place the crucible, without its lid, on the clay triangle. Heat the crucible slowly until the most of the water
evaporates. After most of the water has been removed heat strongly for 3 to 4 minutes. CAUTION: Heat slowly
so no product is lost.
12. Find the combined mass of the crucible, crucible lid, and magnesium oxide. Record in the DATA TABLE.
DATA TABLE:
Mass (g)
Mass of crucible and lid (may want to record each)
Mass of the crucible, crucible lid, and the magnesium
Mass of the crucible, crucible lid, and magnesium oxide (DAY 2)
Mass of the crucible, crucible lid, and magnesium oxide (DAY 3)
CALCULATIONS:
1. Determine the mass of magnesium ribbon used in the experiment.
2. Determine the number of moles of magnesium used.
3. Determine the mass of magnesium oxide.
4. Determine the mass of oxygen that combined with the magnesium.
5. Determine the number of moles of oxygen that were used.
6. Calculate the ratio between moles of magnesium used and moles of oxygen used. Round your answer to the
nearest whole number, since we do not use part of an atom.
7. Give the empirical formula for magnesium oxide that is indicated by your experimental data.
CONCLUSION:
Summarize your results and discuss possible sources of error and their effects!
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LAB 14
DETERMINATION OF AN EMPIRICAL FORMULA
Grading Protocol
Your Name: ________________________
Item
Name & Partners
Partner #1:
Partner #2:
Partner #3:
Partner #4:
Date Experiment Started
Title & # of Experiment
Prelab Question
Appropriate Purpose
Data Tables
Calculations Present & Appropriate
Summary:
Summary of Results
Error Analysis:
Sources of Error
Effects of Errors
TOTAL
Points/Out of
/1
/1
/1
/20
/2
/6
/27
/3
/2
/2
/65
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