02/16/16
CHEM 4
ANSWER KEY
Exam 1: Ch. 2, 3, 5, 6.
Name: __________________________________
Open-End Questions and Problems
Please read the following. To receive full credit for a question or a problem, in addition to the
correct answer, you must show a neat, complete, and logical method of solution where each
number is labeled with the appropriate unit and the final answer is rounded to the correct
number of significant digits. The correct answer without any work shown will generally get
zero credit! When an explanation is required, it should be brief, but accurate and complete.
There are 10 questions for a total of 100 points. (Exam 1 part with multiple choice questions is
worth 50 points. Both parts combined are 150 points total.
1. (10 points) Define the following terms.
Atom is the smallest particle of a chemical element; an electrically neutral
particle made up of protons and neutrons located inside the nucleus and
electrons located outside the nucleus.
Molecule is the smallest particle of a chemical substance; a group of atoms held
together by chemical bonds that carries the total electric charge of zero.
Element (elementary substance) is a chemical substance made up of atoms each
of which contains the same specific number of protons in the nucleus.
Compound is a chemical substance made up of atoms of at least two
different elements.
Mixture is a material of variable chemical composition; it is made up of particles
of at least two different chemical substances.
2. (8 points) The box below schematically represents of a sample of matter. Fill the blanks.
77
The number of atoms in the sample is _______
.
3
The number of elements (types of atom) in the sample is ______
.
28 .
The number of molecules in the sample is _______
4
The number of substances in the sample is _______
.
The number of elements (elementary substances)
1
in the sample is _______
.
3
The number of compounds in the sample is _______
.
liquid and gas
Is the sample gas, liquid or solid? ________________
Is the sample homogeneous
heterogeneous
or heterogeneous? ____________
__________
3. (16 points) Complete the following table.
Chemical Name
Chemical Formula
mercury(II) ion
Hg2+
sulfuric acid
H2SO4
ammonia
NH3
aluminum nitrate
Al(NO3)3
calcium fluoride
CaF2
copper(I) oxide
Cu2O
sulfur trioxide
SO3
ammonium hydrogen phosphate
(NH4)2HPO4
hydrogen peroxide
H2O2
silver ion
Ag+
magnesium hydrogen phosphate
MgHPO4
dichlorine heptaoxide
Cl2O7
iron(II) sulfide
FeS
manganese(II) phosphide
Mn3N2
potassium hydrogen carbonate
KHCO3
potassium hydroxide
KOH
4. (10 points) Provide a term or a name that matches each of the word descriptions below.
An isotope whose atom is currently used as a
standard in defining the atomic mass unit.
carbon-12
The average mass of all atoms of a particular element
atomic weight
found in nature.
A chemical compound that has no ions in it.
covalent compound
A collection of atoms each of which has in its
nucleus a specific number of protons (the same for
each atom in the collection).
chemical element
A combination of atoms that corresponds to the
formula of an ionic compound or a non-molecular
covalent compound.
formula unit
An electrically neutral group of atoms held together
by covalent bonds acting as the smallest independent
particle of a chemical substance.
molecule
A chemical substance that is made of atoms of at
least two different elements.
chemical compound
A characteristic of an atom that indicates the sum of
the numbers of protons and neutrons in its nucleus.
mass number
A material that has no definite chemical composition
as opposed to a pure substance.
mixture
A group of atoms held together by covalent bonds
and carrying a positive or negative net charge.
polyatomic ion
5. (10 points) Although there is only one naturally occurring isotope of gold, 197Au, the atomic
mass of gold given in the modern periodic table is 196.96655, but not 197. Explain.
197 in 197Au is the mass number which is an exact number. It is defined as the
sum of the numbers of protons and neutrons in the nucleus of one atom of the
isotope.
196.96655, the number given below the element’s symbol in the periodic table,
is the atomic weight of gold in amu which is a measured number. In case of
gold, 196.96655 amu is also the mass of individual gold atoms as there is only
one naturally occurring isotope of gold in nature.
6. (10 points) As elementary substances under normal conditions, both fluorine and chlorine
are gases composed of diatomic molecules: F2(g) and Cl2(g).
What is the molecular weight of fluorine?
37.9968
_________________
amu
What is the molecular weight of chlorine?
70.9054
_________________
amu
What are the actual masses (in amu) of individual F2 molecules? Briefly explain.
37.9968. Fluorine has only one naturally occurring isotope: 19F. The mass of one
19
F atom is 18.9984. There is only one possible type of molecule: 19F2.
What are the actual masses (in amu) of individual Cl2 molecules? Briefly explain.
Chlorine has two naturally occurring isotopes: 35Cl (mass of one atom is
34.9689 amu) and 37Cl (mass of one atom is 36.9659 amu).
There are three possible types Cl2 molecule:
35
Cl2, 35Cl37Cl, and 37Cl2 whose masses are 69.9378 amu, 71.9348, and 73.9318
amu respectively.
7. (6 points) Below are the fragments of two graduated cylinders: a 50-milliter one (on the
left) and a 10-milliliter one (on the right). A student recorded the volumes of the liquid in
the cylinders. However, he made a few mistakes. Correct his mistakes by crossing the
incorrect result and writing the correct one above.
23.0 mL
23.0
3.35 mL
3.4mL
1.00 mL
4.3 mL
4.30mL
1.0mL
8. (10 points) Measure (in centimeters) the length and the width of this sheet of paper.
27.90 cm
Length: ____________
21.53 cm
Width: ____________
Calculate the area of the sheet. Make sure to follow the rules of significant digits and label
each number with an appropriate unit. Show your calculations in the space below.
Area = Length × Width = 27.90 cm × 21.53 cm =
600.7 cm2
Area: ____________
Calculate the perimeter of the sheet. Make sure to follow the rules of significant digits and
label each number with an appropriate unit. Show your calculations in the space below.
Perimeter = 2 × (Length + Width) = 2 × (27.90 cm + 21.53 cm) =
99.04 cm
Perimeter: ____________
9. (10 points) The volume of a cylinder of a height and diameter, H and D correspondingly,
can calculated using the following formula:
π×D2×H
Volume =
4
Calculate the volume of the cylinder shown below, determine the uncertainty associated with
the calculated value, and record the result as “value” ± ”uncertainty” accompanied by the
appropriate unit. Show work.
𝑉𝑚𝑎𝑥
𝜋 × (1.40 𝑐𝑚)2 × 14.1 𝑐𝑚
=
= 21.70526 𝑐𝑚3
4
𝑉𝑚𝑖𝑛
𝜋 × (1.36 𝑐𝑚)2 × 13.9 𝑐𝑚
=
= 20.19215 𝑐𝑚3
4
𝑉𝑚𝑎𝑥 − 𝑉𝑚𝑖𝑛
= 0.7566 𝑐𝑚3 ≈ 0.8 𝑐𝑚3
2
20.9
0.8
cm3
Volume = _________
± _________
_________
(value)
(uncertainty) (unit)
10. (10 points) The Oscar statuette is 13.5 inches high and 5.25 inches wide. The body of the
statuette is made of various nonprecious metals and covered with gold. The total mass of
gold used to cover the statuette is 4 troy ounces. One troy ounce (oz t) is defined as exactly
31.1034768 grams. If all the gold were scraped from the statuette and made into a single
cube, what would be the edge of the cube in centimeters? Show work.
𝑀𝑎𝑠𝑠 = 4.00 𝑜𝑧 𝑡 × (
𝑉𝑜𝑙𝑢𝑚𝑒 =
31.103 𝑔
) = 124 𝑔
1 𝑜𝑧 𝑡
𝑀𝑎𝑠𝑠
124 𝑔
=
= 6.45 𝑐𝑚3
3
𝐷𝑒𝑛𝑠𝑖𝑡𝑦 19.3 𝑔/𝑐𝑚
𝑉𝑜𝑙𝑢𝑚𝑒 = 𝐸𝑑𝑔𝑒 3
3
3
𝐸𝑑𝑔𝑒 = √𝑉𝑜𝑙𝑢𝑚𝑒 = √6.45 𝑐𝑚3 = 1.86 𝑐𝑚