CHEM 103 Roxbury Community College Moles and Chemical Formulas Lab Report

REPORT SHEETMoles and Chemical Formulas
LAB
Exp3
Finding the Simplest Formula
1. Mass of empty crucible + cover
38.2337
g
2. Initial appearance of the magnesium
Silver color
g
3. Mass of crucible + cover + magnesium
38.4285
g
4. Mass of crucible + cover + oxide product
38.5578
g
Calculations
5. Mass of magnesium
g
6. Mass of magnesium compound
g
7. Mass of oxygen in the product
g
8. Moles of Mg
(Show calculations.)
mole
9. Moles of O
(Show calculations.)
mole
10. Which number of moles (Mg or O) is smaller
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moles of Mg
=
number)
moles of Mg (rounded to a whole
moles O
=
number)
moles of O (rounded to a whole
Formula:
Mg
O
subscripts
Questions and Problems
Q1 Using the rules for writing the formulas of ionic compounds, write the ions and
the correct formula for magnesium oxide.
Q2 Write a balanced equation for the reaction of the magnesium and the oxygen
(O2 ), including their physical states.
Q3 Calculate the simplest formula for each of the following compounds:
a. 0.200 mole of Al and 0.600 mole of Cl
b. 0.080 mole of Ba, 0.080 mole of S, 0.320 mole of O
Q4 When 2.50 g of copper reacts with oxygen, the copper oxide compound has a
mass of 2.81 g. What is the simplest formula of the copper oxide?
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Exp. 3 Moles and Chemical Formulas
LABORATORY GOALS
• Determine the simplest formula of a compound.
• Calculate the percent water in a hydrate.
• Determine the formula of a hydrate.
Related Topics:
from moles
Formulas, moles, molar mass, calculating moles from grams, calculating grams
CHEMICAL CONCEPTS
Finding the Simplest Formula
The simplest formula of a compound is the lowest whole-number ratio of the atoms in the formula. For
example, the compound benzene, with molecular formula C 6H6 , has the simplest formula CH. Some
molecular formulas and their simplest formulas are shown in Table 11.1.
TABLE 11.1 Examples of Molecular and Empirical Formulas
Name
Molecular Formula
Simplest Formula
Acetylene
C2 H2
CH
Benzene
C6 H6
CH
Ammonia
NH3
NH3
Hydrazine
N2H4
NH2
The simplest formula of a compound is determined by converting the number of grams of each element
to moles and finding the lowest whole-number ratio to use as subscripts. For example, in an experiment it
was determined that 0.040 mole of Zn had combined with 0.080 mole of Cl to form a compound. To
calculate the simplest formula we proceed as follows:
1.
Divide the moles of each element by the smaller number of moles (0.040) and round to the nearest
whole number.
0.080 mole Cl
=2 mole f Cl
0.040
0.040 mole Zn
=1 mole of Zn
0.040
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2.
Use the whole numbers as subscripts to write the formula of the compound.
ZnCl2 (The subscript 1 for Zn is understood.)
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EXPERIMENTAL PROCEDURES
GOGGLES REQUIRED!
Finding the Simplest Formula
Link to the experimental demonstration:

Materials: Crucible, crucible cover, crucible tongs, clay triangle, iron ring and stand, Bunsen burner,
magnesium ribbon, steel wool, eyedropper, small 100- or 150-mL beaker, heat-resistant pad
In this experiment, you will heat magnesium so that it reacts with the oxygen (O2 ) in the air and forms an
oxide. The difference between the mass of the oxide compound and the initial mass of the magnesium is
the mass of oxygen that combined with magnesium. When the moles of the magnesium and the oxygen are calculated, the simplest formula can be determined.
1.
Obtain a clean, dry crucible and its cover. Set the crucible, and cover, slightly offset, on a clay
triangle and place on an iron ring attached to a ring stand (see Figure 11.1). Heat the crucible and
cover for about one minute. Cool until they are at room temperature. Using crucible tongs, carry the
crucible and cover to the balance. Do not place hot objects on a balance pan.
Weigh the crucible and cover and record the mass.
◄ FIGURE 11.1 A crucible and cover, slightly offset, are
heated on a clay triangle.
2.
Obtain a piece of magnesium ribbon that has a mass of 0.15–0.30 g. If there is tarnish on the ribbon,
remove it by polishing the ribbon with steel wool. Describe the appearance of the magne- sium
ribbon after polishing.
3.
Twist the ribbon into a coil and place it at the bottom of the crucible. Weigh the crucible, cover, and
magnesium ribbon and record the mass.
Heating the Magnesium Ribbon
Do this part of the experiment in a fume hood:
• Place the crucible with the magnesium ribbon on the clay triangle. Keep the cover and a pair of
tongs nearby.
• Begin to heat the crucible making sure that the tip of the inner blue flame touches the bottom of
the crucible. The bottom of the crucible will become red hot. Watch for smoke or fumes, which
indicate that the magnesium and oxygen are reacting.
• As soon as the magnesium bursts into flame, use the tongs to place the cover on the crucible. The
cover should be slightly offset to allow oxygen to react with the magnesium. Caution: Avoid
looking directly at the bright flame of the burning magnesium.
• When the magnesium no longer produces smoke or a flame, remove the cover and set it on a heatresistant surface.
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• Continue to heat the crucible strongly for another five minutes. Then turn off the burner and allow
the crucible and its contents to cool to room temperature.
During heating, some magnesium reacts with nitrogen in the air to form magnesium nitride.
3Mg(s) + N2 (g)
Mg3 N2 (s)
To remove this nitride product, carefully add 15–20 drops of water to the cooled contents.
Mg3 N2 (s) + 3H2O(l)
3MgO(s) +2NH3 (g)
Caution: Avoid breathing fumes from the crucible because ammonia may be released.
4.
Cover the crucible and heat gently for five minutes to drive off any excess water. Then heat
strongly for five minutes. Allow the crucible to cool completely.
Reweigh the crucible, cover, and oxide contents and record the mass.
Remove the solid in the crucible and dispose of it as directed by your instructor.
Calculations
5.
Determine the mass of the magnesium (3 – 1).
6.
Calculate the mass of the magnesium compound (4 – 1).
7.
Calculate the mass of oxygen that combined with the magnesium (6 – 5).
8.
Determine the number of moles of magnesium by dividing the mass of magnesium (5) by its
molar mass.
moles of Mg = g Mg ×
9.
Determine the number of moles of oxygen by dividing the mass of the oxygen (7) by its molar
mass.
moles of O = g O ×
10.
11.
1 mole Mg
24.31 g Mg
1 mole O
16.00 g O
Divide the moles of Mg (8) and the moles of O (9) by the smaller number of moles. Round each of
the results to the nearest whole number.
Using the whole number values obtained in 10 as subscripts, write the simplest formula of the
magnesium compound.
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