Intro to Chemistry Chem1020 Lab
Empirical Formula
Chemistry Department
Minneapolis Community & Technical College
1
Overview
• Part I
Introduction
• Part II
Analytical balance
• Part III
Bunsen burner
• Part IV
Procedure
• Part V
Calculation
2
Part I. Introduction
• The goal of this experiment is to determine the empirical formula
of the compound made of magnesium (Mg) and oxygen (O). This
topic is discussed in Chapter 8.7 in the textbook.
• You will start with a Mg ribbon, which is transformed into its
oxygen compound by burning over a Bunsen burner. A crucible
will be used for the high temperature the reaction requires.
• You will need to take three accurate mass measurements done on
an analytical balance:
1. Crucible
2. Crucible + Mg ribbon
+
3. Crucible + compound MgxOy
+
3
Part II. Analytical Balance
An analytical balance is much more
accurate than a top-loading balance.
This high accuracy, however, comes
with a much higher price.
So, please take extra care. Thanks!
1. Always use the same balance.
2. Always have a piece of weighing
paper on the balance.
3. Before putting your object on the
balance, press the “Zero” or “0/T”
button to zero it.
4. Gently slide open a side-door and
put the object on the balance.
Close the door immediately.
sliding
side-door
“zero”
button
5. Record all digits shown in the
display (including zeros) with the
unit of “g”. Example: this
crucible is 21.888 ± 0.001 g.
6. Never put a hot object on the
balance! Never try to remove the
balance!
4
Part III. Bunsen Burner
Closed
1. Connect the burner and the
gas valve at your station with a
rubber hose. Make sure the
gas valve at your station is at
the closed position.
2. Gently close the gas valve
of the burner.
5. Approach a striker
to the top of the
barrel.
Open
4. Open the gas valve
at your station.
Meanwhile quickly
turn on the gas valve
of the burner.
3. Gently close the air valve
of the burner.
Hottest spot
6. Turn on
the air valve
until two
blue cones
form in the
flame.
5
Part IV. Procedure
Mg ribbon
1. Bring the following items to a weighing station:
1. Sand .it over a trash
bin till it looks shiny.
Mg ribbon
2. Wrap it around a
pen to make it curly.
2. Use a pair of tongs to
transfer the crucible from the
beaker onto the balance.
Record the mass of crucible.
3. Without removing the
crucible, put the Mg ribbon
into the crucible. Record
their total mass.
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4. Put the crucible (with the Mg inside) on
the clay triangle. Position the lid so that
it is partially open and supported by clay
triangle.
5. Light up the Bunsen
burner away from the
crucible. Adjust the gas
valve so that the blue
inner cone is about 1 in.
tall.
Attention: Crucibles tend to crack under sudden temperature changes. Heat it up
gradually and cool it down gradually as well.
7. Adjust the gas valve so that the tip of the
6. Hold the base of the
blue inner cone just touches the bottom
burner and use the outer
of the crucible. After a few minutes,
blue cone to heat the
peek the inside of the crucible where Mg
crucible in a circular
is reacting with both O2 and N2 in the
motion. After ~ 1 min.,
air. Record the observations.
put the burner right
underneath the crucible
and heat for another
minute or so still with
the blue outer cone.
7
8. After a total of 10 min.
heating, stop heating
gradually: adjust the gas
valve so that the inner
cone is about 1 in. tall; 1
min later, hold the burner
and heat the crucible for
~ 1 min. in a circular
motion, with the outer
blue cone of the flame.
10. Inside the crucible, it is
a mixture of
magnesium oxide and
magnesium nitride.
When no heat is felt
radiating from the
crucible, add 10 drops
of distilled water to
convert nitride into
oxide. Use the probe
provided to lift the lid.
9. Stop the burner by
turning off the gas
valve at station
only. Do not make
any adjustment on
any valves on the
burner itself.
11. Light up the Bunsen
burner by turning on
the gas valve at station.
No adjustment on the
burner should be
necessary. Repeat the
heating/cooling
procedure in steps 6-8.
Five minutes of heating
is sufficient.
12. When the crucible is completely
cooled down, determine and record
its mass with the final ashy product.
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Part V. Sample Calculation
Data:
a. Mcrucible
21.888 ± 0.001 g
b. Mcrucible+Mg 22.076 ± 0.001 g
c. Mcrucible+compound 22.201 ± 0.001 g
Calculation (You must show how you get each result and follow sig. fig. rules):
a. MMg = Mcrucible+Mg – Mcrucible = 22.076 – 21.888 = 0.188 g (3 decimal places)
b. Mcompound = Mcrucible+compound – Mcrucible = 22.201 – 21.888 = 0.313 g (3 decimal places)
c. Moxygen = Mcompound - MMg= 0.313 – 0.188 = 0.125 g (3 decimal places)
d. mole of Mg  M Mg 
1
1mole
 0.188 g 
 0.00773mole Mg (3 sig. fig.)
molar mass of Mg
24.31g
e. mole of O  M oxygen 
1
1mole
 0.125 g 
 0.00781mole O (3 sig. fig.)
molar mass of O
16.00 g
f. Ratio of atoms  mole of Mg : mole of O  0.00773 : 0.00781 
g. Empirical formula of the compound: Mg1O1.01 = MgO
0.00773 0.00781
:
 1 : 1.01
0.00773 0.00773
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