Name: _______________________________________
Group: ______________
EST
Questions 1–33 and A–C
Checkups and follow-ups
CHAPTER 2
Date: ___________
ANSWER KEY
Molecules and solutions
Checkup
1 WHAT IS A MOLECULE? (pp. 40–50)
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1. Are the following substances molecules? Explain your answer.
a) NaCl
Yes, it is a molecule because it contains two different atoms: sodium and chlorine.
b) O2
Yes, it is a molecule because it contains two oxygen atoms.
c) Au
No, it is not a molecule; it is only one atom: gold.
d) Co
No, it is not a molecule; it is only one atom: cobalt.
e) CO
Yes, it is a molecule because it contains two different atoms: carbon and oxygen.
f) H2SO4
Yes, it is a molecule because it contains several different atoms: two hydrogen atoms,
one sulphur atom and four oxygen atoms.
2. What is the natural tendency of each of the following elements with regard to gaining or losing
electrons?
a) potassium
Potassium tends to lose an electron.
b) oxygen
Oxygen tends to gain two electrons.
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c) aluminum
Aluminum tends to lose three electrons.
d) krypton
Krypton is a noble gas, so it does not tend to gain or lose electrons.
3. Metals give up electrons. Consequently, do they tend to form positive or negative ions?
They tend to form positive ions.
4. Calculate the sum of the charges for each of the following electron transfers, and indicate the
net charge of the resulting ion.
a) A calcium atom loses two
electrons.
c) An iodine atom gains one electron.
(+20) protons
(+53) protons
+ (–18) electrons
+ (–54) electrons
–1
An I – ion is obtained.
+2
A Ca ion is obtained.
2+
b) A nitrogen atom gains three electrons.
d) An aluminum atom loses three electrons.
(+7) protons
(+13) protons
+ (–10) electrons
+ (–10) electrons
–3
An N
3−
+3
3+
ion is obtained.
An Al ion is obtained.
5. Do the models below represent atoms or ions? Explain your answers.
b)
It is an atom because it contains an equal
It is an ion because it contains more protons
number of protons and electrons.
than electrons.
6. Iron has the ability to form one of two ions: Fe2+ or Fe3+. How many protons and electrons does
each of these ions contain?
Fe2+: 26 protons and 24 electrons
Fe3+: 26 protons and 23 electrons
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2
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a)
Name: _______________________________________
Group: ______________
Date: ___________
7. Various soaps and detergents contain phosphates that contaminate the water in lakes and
rivers. For this reason, many environmental groups encourage consumers to use phosphatefree cleansers.
a) Is a phosphate an atom, a molecule or an ion?
A phosphate is a polyatomic ion.
b) What is its chemical formula?
Its chemical formula is PO43–.
8. Do the following molecules contain a polyatomic ion? If so, write its chemical formula and its
name according to the rules of nomenclature.
a) NaOH
Yes. Chemical formula: OH −. Name: hydroxide ion
b) C6H22O11 No, it does not contain a polyatomic ion
c) MgSO4
Yes. Chemical formula: SO42−. Name: sulphate ion
d) H3PO4
Yes. Chemical formula: PO43−. Name: phosphate ion
9. Which of the following substances contain at last one ionic bond?
HCl, NaOH, CO2, KBr, CaO, AlCl3, NH3
NaOH, KBr, CaO, AlCl3
10. Draw a Lewis structure for each of the following molecules to show how their ionic bonds are
formed.
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a) LiF
b) CaBr2
c) Li3N
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d) MgO
11. What is the charge of each of the ions in the following molecules?
a) NaBr Na+ and Br−
b) CaO
Ca2+ and O2−
c) Li2S
Li+ and S2−
d) AlF3
Al3+ and F−
12. Which of the following substances contain at least one covalent bond?
CH3COOH, CS2, CoCl2, Fe2O3, H2O, CH4, AgCl
CH3COOH, CS2, H2O, CH4
13. Draw a Lewis structure for each of the following molecules to show how their covalent bonds
are formed.
c) CO2
b) CBr4
d) HF
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a) Cl2
14. What does the subscript 2 mean in the molecule MgCl2?
It means that this molecule contains two chlorine atoms.
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4
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15. Write the chemical formula of the molecule resulting from the union of each of the following
pairs of substances.
a) potassium and sulphur
K2S
b) chlorine and copper
CuCl2
c) chromium and fluorine
CrF3
d) the ions Mg
2+
and
SO42−
MgSO4
16. Apply the rules of nomenclature to name the following substances.
a) NaBr Sodium bromide
b) PCl5
Phosphorus pentachloride
c) SiCl4
Silicon tetrachloride
d) K3N
Potassium nitride
e) Al2O3 Aluminum oxide
2 PROPERTIES OF SOLUTIONS (pp. 50–61)
17. Name the substances that act as solvents and solutes in each of the following solutions.
Solution
a) Air is composed of approximately
80 percent nitrogen and 20 percent
oxygen.
b) Steel is an alloy of iron with a small
amount of carbon.
Solvent
Solute
Nitrogen
Oxygen
Iron
Carbon
Water
Mineral salts
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c) Seawater contains many mineral salts.
18. Fruit juices, body fluids and cleaning solutions for contact lenses are all examples of aqueous
solutions. What is the solvent in these solutions?
In aqueous solutions, the solvent is water.
19. Brine is a solution of sodium chloride (NaCl) at 18% m/V, which is used to preserve certain
foods. To prepare 250 mL of brine, how much salt must be dissolved?
Problem-solving steps
1. Determine what you are
looking for.
Observatory / Guide
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Application
The mass of salt needed to prepare 250 mL of brine
at 18% m/V
5
ANSWER KEY
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Name: _______________________________________
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Date: ___________
2. Determine the different
variables and their respective
values.
C = 18% m/V
V = 250 mL
m=?g
3. Choose the appropriate
formula.
C= m
V
4. Replace each variable by its
value and isolate the unknown
variable.
18 g
?g
=
100 mL 250 mL
Therefore m = 45 g
5. Check your solution and
answer the problem question.
The amount of salt needed to prepare 250 mL of brine at
18% m/V is 45 g.
20. A technician prepares a solution, following the steps illustrated below. What is the
concentration of this solution, in g/L?
Application
1. Determine what you are
looking for.
The concentration of the solution in g/L
2. Determine the different
variables and their respective
values.
C = ? g/L
V = 80 mL = 0.08 L
m = 10 g
3. Choose the appropriate
formula.
C= m
V
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Molecules and solutions
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4. Replace each variable by its
value and isolate the unknown
variable.
5. Check your solution and
answer the problem question.
C=
Group: ______________
Date: ___________
10 g
= 125 g/L
0.08 L
The concentration of the solution is 125 g/L.
21. The label on a bottle of water says that the water contains 45 ppm of sodium.
a) What does this mean?
It means that the water contains 45 parts of sodium per million parts of solution.
b) What is the concentration of sodium in g/L?
45 ppm =
45 mg
1L
=
0.045 g
1L
The sodium concentration is 0.045 g/L.
22. The water in a lake is contaminated. To determine the concentration of the contaminant,
a technician takes a 50-mL sample of the water. After several tests, he concludes that the
sample contains 3.75 mg of contaminant. Calculate the concentration of the contaminant,
in ppm.
3.75 mg
? mg
=
= 75 mg/L = 75 ppm
50 mL
1000 mL
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The concentration of the contaminant is 75 ppm.
23. Certain minerals are essential to good health. For example, a person should take in
approximately 350 mg of magnesium daily. Magnesium can be found in many foods, such
as whole wheat bread, which contains about 850 ppm. Assuming that a slice of whole wheat
bread has a mass of about 30 g, how many would a person have to eat to obtain the
recommended daily dose of magnesium? Show your calculations.
850 ppm =
850 mg
=
1000 g
350 mg
?g
411.8 g
= 13.73
30 g
A person would have to eat approximately 14 slices of bread to obtain the recommended daily
dose of magnesium.
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24. A technician dissolves 50 g of potassium nitrate (KNO3) in a small amount of water and then
adds more water to obtain 500 mL of solution. What is the molar concentration of the solution?
Application
Problem-solving steps
1. Determine what you are
looking for.
The molar concentration of the solution
2. Determine the different
variables and their
respective values.
C = ? mol/L
V = 500 mL = 0.5 L
m = 50 g
n = ? mol
M = 39.10 + 14.01 + (3  16.00)
= 101.11 g/mol
3. Choose the appropriate
formulas.
M=
101.11 g/mol =
n=
C=
5. Check your solution and
answer the problem question.
50 g
? mol
50 g
= 0.5 mol
101.11 g/mol
0.5 mol
= 1 mol/L
0.5 L
=
The molar concentration of the solution is 1 mol/L.
25. For the purposes of an experiment, a student must take a sample of exactly 0.2 mol of copper
sulphate (CuSO4) from a 0.5 mol/L solution. What volume of solution must she measure?
0.5 mol
0.2 mol
=
1L
?L
She must measure 0.4 L, or 400 mL.
CHAPTER 2
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8
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4. Replace each variable by its
value and isolate the
unknown variable.
m
n
C= n
V
Name: _______________________________________
Group: ______________
Date: ___________
26. A chemist must prepare 300 mL of a 1.5 mol/L solution of magnesium phosphate
(Mg3(PO4)2). What mass of the solute must she measure?
1.5 mol
=
1L
? mol
0.3 L
0.45 mol of magnesium phosphate
?g
0.45 mol
=
262.87 g
1 mol
She must measure 118.3 g of magnesium phosphate.
27. Which of the beakers below probably contains an electrolyte? Explain your answer.
The beaker containing the calcium sulphate,
because the light is on, which means that
current is flowing through the solution.
28. How does a solution conduct electricity?
Through the presence of dissolved ions. The ions
are electrically charged and, in the presence of
electric current, they start to move. The
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current can then flow through the solution.
29. Are the following equations of electrolytic dissociation written correctly? Explain your
answers.
a) MgO(s)
Mg2+(aq) + O2−(aq)
The equation is written correctly. There are two oppositely charged ions, and the sum of their
charges is neutral.
Ca2+(aq) + Br−(aq)
b) CaBr2(s)
The equation is not written correctly because the sum of the charges of the ions is not neutral.
c) LiH(s)
Li+(aq) + H+(aq)
The equation is not written correctly because both ions are positively charged.
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Date: ___________
2 Al3+(aq) + 3 O2− (aq)
d) Al2O3(s)
The equation is written correctly. There are two oppositely charged ions, and the sum of their
charges is neutral.
30. What distinguishes a strong electrolyte from a weak one?
A strong electrolyte dissociates completely into ions when it is in solution, while a weak electrolyte
only partially dissociates into ions.
31. Is each of the following substances an acid, a base or a salt?
a) HBr
An acid
d) ZnSO4
A salt
b) KOH
A base
e) H3PO4
An acid
c) BaF2
A salt
f) Cu(OH)2 A base
32. Look at the photo opposite.
a) What is the pH of this solution?
The solution has a pH of 2.
b) Is the solution acidic, basic or neutral?
Explain your answer.
The solution is acidic because its pH is less
than 7.
33. Rain has a pH of about 5, while the pH of seawater is about 8. How many times more acidic
is rainwater than seawater?
Rain is 1000 times more acidic than seawater.
A. A coalition was formed recently to protest the use of dihydrogen monoxide. According to the
coalition, the widespread use of this substance has harmful effects on human health and the
environment.
Dihydrogen monoxide is both colourless and odourless. It exists as a solid, liquid and gas, but
it is usually sold in liquid form. Certain precautions must be taken during its use. Its vapour can
cause burns, while prolonged exposure to the compound in its solid form can cause frostbite.
Inhaling it in its liquid form has resulted in many deaths.
a) What is the chemical formula for dihydrogen monoxide?
H2O
b) Dihydrogen monoxide is the main component of acid rain. Is it the solute or the solvent?
Explain your answer.
It is the solvent because it represents the greater part of the solution.
CHAPTER 2
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10
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REVIEW QUESTIONS
Name: _______________________________________
Group: ______________
Date: ___________
c) In its pure state, this substance does not conduct electricity, yet it is used to manufacture
batteries. Explain how this is possible.
The electrolytes dissolved in the substance can carry an electric current.
d) Soil erosion by this substance dissolves the minerals in the soil and carries them into lakes
and rivers. How does this affect the concentration of minerals in the lakes and rivers?
Under these conditions, the mineral content increases.
e) If the total concentration of dissolved matter in a lake is 500 mg/L, what is its concentration
in ppm?
Given that
1 mg/L = 1 ppm
500 mg/L = 500 ppm
f) Should the use of dihydrogen monoxide be banned? Explain your answer.
No, because dihydrogen monoxide is, in fact, water.
B. Are the atoms in dihydrogen monoxide joined by ionic or covalent bonds? Explain your
answer.
They are joined by covalent bonds because hydrogen and oxygen are both nonmetals.
C. Prepare your own summary of Chapter 2 by building a concept map.
See the Concept maps section in Guide B.
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Follow-up
1. To fight air pollution, we must reduce our reliance on fossil fuel combustion. What can each
person do to contribute to this reduction?
Answers will vary. Examples: We can choose a means of transportation that uses little or no fossil
fuels, such as walking, biking or taking public transit.
– As much as possible, we can prefer local products to items that are shipped long distances by truck or
boat.
2. Describe some consequences of acid rain in your region.
Answers will vary. Examples: Acid rain has a negative impact on maple syrup production because it
damages maple trees.
– It damages stone buildings.
– It affects the health of fish.
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