1. No category

the amount in moles of silver atoms formed. Describe the procedure, materials, and equipment used, safety procedures, and
explain the calculations needed.
Making Connections
7. Suppose that there is a prestigious award given by the Academy
of Science each year to the most significant scientific concept.
Write a paper nominating the mole concept for this award, citing
the mole’s role and importance in the application of chemical
reactions in society, industry, and the environment.
Percentage Composition
4.5
Figure 1
This experimental car burns hydrogen as a
fuel, producing water vapour as an exhaust.
The dish collects solar energy, which is used
to dissociate water into hydrogen and
oxygen.
Using molar mass values from a periodic table, we can calculate the mass of reactants and products in chemical reactions—if we already know the chemical formulas. However, when a new substance is produced, we first need to determine its
chemical formula. To do this, we need to determine the chemical composition of the
compound, that is, what elements it is made of and the quantities of each element.
In this section, we will experimentally determine the composition by mass of
a substance and then convert the mass amounts to percentages, to give us the
percentage composition. We can then use atomic mass and molar mass to determine the correct chemical formula.
Before we begin, we will practise the mathematical steps in the calculation of
percentage composition from mass measurements of reactants and products.
Let’s consider 20 g of red jelly beans mixed with 30 g of green jelly beans. What
is the percentage composition of the mixture, by mass? The percentage of red
jelly beans by mass would be 20 g of the total 50 g, which is 20 g/50 g × 100%, or
40%. Similarly, the percentage of green jelly beans would be 30 g of the total 50 g,
which is 60%.
Now let’s look at the percentage composition of water. Water is formed when
hydrogen is allowed to react with oxygen, a reaction that gives off large amounts
of energy (Figure 1). The results of an experiment revealed that 2.5 g of
hydrogen, when completely reacted, produced 22.5 g of water. What is the percentage composition of water by mass?
Since 2.5 g of hydrogen combined with an amount of oxygen to produce
22.5 g of water, we can calculate the mass of oxygen by subtraction:
2.5 g
mH
mH O 22.5 g
2
mO
(22.5 – 2.5) g 20.0 g
mH
× 100%
%H mH
O
2
2.5 g
% H × 100% 11.1%
22.5 g
Similarly,
20.0 g
% O × 100%
22.5 g
% O 88.9%
178 Chapter 4
4.5
We will see later how this percentage composition, together with the atomic mass
of hydrogen and oxygen, will allow us to determine a possible formula for water.
Sample Problem 1
Sodium is a very reactive alkali metal, and chlorine is a poisonous green gas.
When they are allowed to react, they combine to form sodium chloride, the
stable and usually harmless table salt, an ionic compound. In an experiment,
3.45 g of sodium metal reacted with 5.33 g of chlorine gas to give 8.78 g of
sodium chloride. Calculate the percentage composition by mass of sodium chloride.
Solution
mNa+ 3.45 g
mCl 5.33 g
mNaCl 8.78 g
3.45 g
% Na+ × 100%
8.78 g
% Na+ 39.3%
5.33 g
% Cl × 100%
8.78 g
% Cl 60.7%
Therefore, the percentage composition by mass of sodium chloride is 39.3%
sodium and 60.7% chlorine. (Note that the two percentages total 100%.)
Practice
Understanding Concepts
1. A 27.0-g sample of a compound contains 7.20 g of carbon, 2.20 g of
hydrogen, and 17.6 g of oxygen. Calculate the percentage composition of the compound.
2. Carbon will burn in sufficient oxygen to produce carbon dioxide. In
an experiment, 8.40 g of carbon reacts with oxygen and 30.80 g of
carbon dioxide is produced.
(a) What mass of oxygen reacted with 8.40 g of carbon?
(b) Calculate the percentage composition by mass of carbon dioxide.
Answers
1. 26.7%, 8.1%, 65.2%
2. (a) 22.40 g
(b) 27.3% C, 72.7% O
3. (a) 80.1% Cu, 19.9% O; 88.9%
Cu, 11.1% O
3. In one sample of a compound of copper and oxygen, 3.12 g of the
compound contains 2.50 g of copper and the remainder is oxygen. In
another sample of a compound of copper and oxygen, 1.62 g of the
compound contains 1.44 g of copper and the remainder is oxygen.
(a) Calculate the percentage composition of each compound.
(b) Are the two samples of the same compound? Give reasons for
your answer.
Quantities in Chemical Formulas 179
INQUIRY SKILLS
Questioning
Hypothesizing
Predicting
Planning
Conducting
Recording
Analyzing
Evaluating
Communicating
Wear eye protection.
Use crucible tongs to
transfer hot crucible.
Care is required in using a
laboratory burner and handling hot apparatus.
Magnesium ribbon burns
with a hot flame and an
extremely bright light. If the
magnesium ignites, do not
look directly at the flame
because it may damage your
eyes.
Investigation 4.5.1
Percentage Composition by Mass of Magnesium
Oxide
In this investigation, you will test the law of definite proportions. To do this, you
will determine the composition by mass of magnesium oxide, calculate the percentage composition, and compare your results and those of other students to
the predicted values.
Magnesium is a silvery metal that burns with such a bright flame that it was
once used in flashbulbs for photography. Magnesium oxide, a white powder, is
produced as a result of the synthesis reaction of combusting magnesium. Here,
you will conduct a slow combustion of magnesium and then complete the
Evidence, Analysis, and Evaluation sections of a lab report.
Questions
What is the percentage composition by mass of magnesium oxide?
Is this percentage constant?
Prediction
(a) Make a prediction based on the law of definite proportions. What should
the percentage composition by mass of magnesium oxide be?
Experimental Design
DID YOU KNOW ?
Magnesium Sparkles
Finely ground magnesium powder readily
ignites upon heating in air, giving a dazzling
white light. This property of magnesium is
used in devices such as flares, incendiary
bombs, and fireworks. Magnesium dust is a
common component of fireworks, including
sparklers, a nonexplosive type of fireworks
constructed by coating a slurry of chemicals
on a wire. The slurry consists of a fuel containing magnesium powder, an oxidizer, iron
or steel powder, and a binder, all of which
are proportioned to burn slowly, giving off a
shower of bright shimmering sparks.
180 Chapter 4
A known mass of magnesium metal is heated in a crucible over a laboratory
burner. The mass of the magnesium oxide formed is used to determine the mass
of the oxygen that reacted and the percentage composition by mass of the two
components. Some of the magnesium also reacts with nitrogen in air to form a
nitride. This compound is converted to magnesium oxide by adding water and
reheating the solid.
Materials
lab apron
eye protection
centigram or analytical balance
7–8 cm magnesium ribbon
steel wool
porcelain crucible and lid
laboratory burner
retort stand
ring stand and clamp
clay triangle
crucible tongs
glass stirring rod
distilled water
4.5
Procedure
1. Using a balance, determine the mass of a clean, dry porcelain crucible and
lid.
2. Polish the magnesium ribbon with the steel wool and fold the ribbon to fit
into the bottom of the crucible.
3. Determine the mass of the crucible, lid, and magnesium ribbon.
4. Place the crucible securely on the clay triangle. Set the lid slightly off-centre
on the crucible to allow air to enter but to prevent the magnesium oxide
from escaping.
5. Place the laboratory burner under the crucible, light it, and begin heating
with a gentle flame.
6. Gradually increase the flame intensity until all the magnesium turns into a
white powder.
7. Cut the flow of gas to the burner and allow the crucible, lid, and contents
to cool.
8. Using the stirring rod, crush the contents of the crucible into a fine
powder. Carefully add about 10 mL of distilled water to the powder. Use
some of the water to rinse any powder on the stirring rod into the crucible.
9. Heat the crucible and contents, with the lid slightly ajar, gently for 3 min
and strongly for another 7 min.
10. Allow the crucible and contents to cool.
11. Using the balance, determine the mass of the cooled crucible, lid, and contents.
Figure 2
Apparatus for heating in a crucible
Analysis
(b) What evidence do you have that a chemical reaction took place?
(c) Calculate the mass of oxygen that reacted with the magnesium.
(d) Use your evidence to calculate the percentage composition by mass of
magnesium oxide.
(e) Based on the evidence (your results and those of your classmates), what are
the answers to the Questions?
Evaluation
(f) If some of the magnesium oxide had escaped from the crucible, would
your percentage composition calculation of magnesium be too high or too
low? Explain.
(g) If the magnesium had reacted with some other component in the air,
would your percentage composition calculation of magnesium be too high
or too low? Explain.
(h) The magnesium ribbon was polished to remove any white film on its surface before beginning the experiment. Explain why this is necessary.
(i) Suggest a modification in the procedure to ensure that all of the magnesium completely reacts with oxygen.
(j) Evaluate your prediction. Based on the evidence obtained from several
groups, is the law of definite proportions valid?
Quantities in Chemical Formulas 181
Practice
Making Connections
4. Use the Internet to research information about tires. When you look
at the sidewall of a tire, you will see a lot of information, such as the
name of the tire, its size, whether it is tubeless or a tube type, the
grade, and the speed rating. It also gives important safety information such as the maximum load and maximum inflation for the tire. In
addition, the composition of the tire can be obtained from the manufacturer. For example, a Goodyear all-season passenger tire contains
approximately
Answers
4. (a) 18.6%, 23.7%, 5.2%, 5.2%,
5.2%, 14.4%, 27.8%
1.8
2.3
0.5
0.5
0.5
1.4
2.7
kg
kg
kg
kg
kg
kg
kg
of
of
of
of
of
of
of
8 types of natural rubber
8 types of carbon black
steel cord for belts
polyester and nylon
steel bead wire
40 kinds of chemicals, waxes, oils, pigments
5 different types of synthetic rubber
(a) Calculate the percentage composition of this tire.
(b) Research and compare the percentages of synthetic and natural
rubber used in various types of tires, for example, tires for light
trucks, racecars, and off-highway vehicles.
(c) Relate the characteristics of synthetic and natural rubber mixes to
their application in the different types of tires.
(d) Research the contributions of Goodyear in the composition of
rubber, and assess the impact of the development of rubber tires
on society and the environment.
Follow the links for Nelson Chemistry 11, 4.5.
GO TO
www.science.nelson.com
Try This
Activity
What Makes Popcorn Pop?
In each kernel of popping corn, there is a small drop of water in a circle
of soft starch. When heated, the water expands and builds up pressure
against the hard outer surface, eventually exploding and turning the
kernel inside out.
Materials: popping corn, hot-air popcorn popper, balance
•
•
•
•
Measure the mass of some unpopped popping corn.
Pop the popping corn.
Allow the popcorn to cool and measure the mass again.
Assume that any difference in masses is caused by loss of water
from the kernels. Calculate the percentage of water in the sample of
popcorn.
• Repeat the activity with kernels of popping corn that have been cut
in half either lengthwise or crosswise.
• Record the percentage of popped kernels from each cutting method.
(a) Do the results confirm the given reason why popcorn pops?
Explain.
182 Chapter 4
4.5
Percentage Composition Calculations from a Formula
We have seen how to determine the percentage composition of a compound
through experimentation. Empirical information is often used to determine the
formula of a compound.
Sometimes we also need to calculate the percentage composition of a compound whose formula we already know. For example, we may wish to verify the
purity of a compound by comparing its percentage composition obtained experimentally to the theoretical value, calculated from the compound’s formula.
Percentage composition also has commercial uses, for example, in fertilizers. The
chemical formula can be used to determine the percentage by mass that is contributed by each element in the compounds that make up the fertilizer. Nitrogen
is one of the key elements delivered to plants by fertilizers; it is important to calculate the percentage of nitrogen in fertilizer compounds to determine the correct quantities of fertilizer to apply.
If we know the chemical formula of the compound, calculating percentage
composition is straightforward. Essentially, we want to calculate the “contribution” of each element to the total mass of the compound. Thus, we first calculate the mass of all the atoms of each element. Then, we calculate the total mass
of all the elements in the compound. To obtain the percentage contribution of
each element, we divide the mass of each element by the total mass.
Sample Problem 2
Determine the percentage composition of sodium carbonate, Na2CO3, also
known as soda ash.
Solution
mNa
22.99 u × 2 45.98 u
mC
12.01 u × 1 12.01 u
mO
16.00 u × 3 48.00 u
mNa CO
2
3(s)
105.99 u
45.98 u
% Na × 100%
105.99 u
% Na 43.38%
12.01 u
% C × 100%
105.99 u
% C 11.33%
48.00 u
% O × 100%
105.99 u
% O 45.29%
The percentage composition of Na2CO3(s) is 43.38% sodium, 11.33% carbon,
and 45.29% oxygen.
Quantities in Chemical Formulas 183
Practice
Understanding Concepts
Answers
5. 2.1% H, 32.7% S, 65.2% O
6. 41.6% Mg
7. 77.7% Fe, 22.3% O; 69.9% Fe,
30.1% O
8. 28.2% N
5. Calculate the percentage composition by mass of sulfuric acid,
H2SO4(aq), used in car batteries.
6. Calculate the percentage by mass of magnesium in magnesium
hydroxide, Mg(OH)2(s), used in some antacids.
7. Iron and oxygen combine to form two different compounds. The formulas of the compounds are FeO(s) and Fe2O3(s). Calculate the percentage composition of each compound.
8. Calculate the percentage of nitrogen in ammonium phosphate,
(NH4)3PO4, a compound used in fertilizers.
Section 4.5 Questions
Understanding Concepts
1. Explain why it is necessary to determine the percentage composition of a new compound by experiment.
2. In a compound consisting of potassium and chlorine, 33.5 g of
potassium combined with 30.4 g of chlorine. Calculate the percentage composition of the compound.
3. The following evidence was obtained in an experiment to determine the percentage composition of a compound containing
sodium, sulfur, and oxygen:
mass of Na atoms 23.0 g
mass of S atoms 16.0 g
mass of O atoms 32.0 g
Calculate the percentage composition of this compound.
4. Ammonium nitrate, NH4NO3(s), and ammonium sulfate,
(NH4)2SO4(s), are both compounds used as fertilizers. Determine
which compound contains the greater percentage by mass of
nitrogen.
5. Calcium sulfate dihydrate, CaSO4•2H2O(s), is commonly called
gypsum and is used in building materials such as drywall. It contains water of crystallization, some of which is lost on heating,
leaving (CaSO4)2•H2O(s). Compare the percentage by mass of
water in each compound.
Applying Inquiry Skills
6. In this lab exercise, a synthesis reaction is used to determine the
percentage composition of a compound of copper and sulfur.
Experimental Design
When heated strongly in a crucible, copper wire or turnings react
with an excess of sulfur to produce a solid, a sulfide of copper.
Procedure
(a) Design a Procedure, based on the Experimental Design, to
obtain the evidence needed.
Analysis
(b) Explain how the evidence gathered in your procedure would
be used to calculate the percentage composition of the
product.
184 Chapter 4
4.6
Evaluation
(c) Evaluate the experimental design.
Making Connections
7. Name two products that you might find around the house where
percentage compositions are given
(a) in mass;
(b) in measurements other than mass.
4.6
Empirical and Molecular
Formulas
If we are given an unknown substance and we want to find its chemical formula,
how do we begin? We need to identify the elements that are in the compound as
well as the number of atoms of each element. To begin with, we can determine
its percentage composition by mass. Then, we simply convert the mass values
into amounts in moles, which gives us the subscripts in the chemical formula of
the substance.
A formula derived in this way is called an empirical formula, which means
that it is derived from observations in an experiment rather than from theory. An
empirical formula tells us the simplest ratio of the combining elements.
Currently, many new substances that are synthesized are organic compounds, that is, contain mainly carbon and hydrogen atoms. One of the technologies available to measure the percentage composition of these compounds
uses combustion analyzers (Figure 1). In this process, several milligrams of a
compound are burned inside a combustion chamber. When the compound is
burned, oxygen combines with the carbon atoms to form carbon dioxide and
with the hydrogen atoms to form water vapour. Any other elements present are
similarly converted to their oxides. The quantities of these combustion products
are precisely measured, and computer analysis discloses the percentage by mass
of each element detected in the compound. This percentage composition is then
used to calculate the empirical formula of the compound.
sample
H2O
absorber
O2
furnace
empirical formula: simplest wholenumber ratio of atoms or ions in a compound
CO2
absorber
Figure 1
A substance burned in a combustion analyzer
produces oxides that are captured by
absorbers in chemical traps. The initial and
final masses of each trap indicate the masses
of the oxides produced. These masses are
then used in the calculation of the percentage
composition of the substance burned.
An empirical formula does not necessarily provide the correct information
about the number of atoms in a molecule. That is, an empirical formula may not
Quantities in Chemical Formulas 185