03/15/17
CHEM 4
Quiz 3 (20 points)
ANSWER KEY
Name
1. (4 points) Provide a term or a name that matches each of the word descriptions below.
The average mass of all atoms of a particular element
found in nature.
atomic weight
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
An electrically neutral group of atoms held together
by chemical 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 subatomic particle residing in the nucleus of the
atom and carrying no electric charge.
neutron
2. (4 points)
(a) Perform the following conversions. (You do not have to show your work.)
0.670
6.70×105 mg =
8.00×1013
8.00×107 µm2 =
kg
nm2
4.00
0.000400 km =
2.70×107 pm3 =
2.70×10−23
dm
cm3
(b) Compare by placing >, <, =, or ≈ between the two entries in each pair.
1 ft3
>
1000 mm3
1 m2
1L
<
1L
>
1000 kg/m3
1 ft2
=
1 g/cm3
10 cm
≈
4 in
1L
=
10 dL
1 g/mL
=
1 kg/L
1 mL
>
100 µL
PLEASE TURN OVER!!!
3. (4 points) The temperature of dry ice (sublimation temperature at normal pressure) is –
109°F. Is this higher or lower than the temperature of boiling ethane (a component of bottled
gas), which is –88°C?
Show all the steps of your work.
°C = (°F − 32) / 1.8 = (−109 − 32) / 1.8 = −78.3°C
−78.3°C
>
−88°C
The temperature of dry ice is higher than the temperature of boiling ethane.
4. (4 points) Although there is only one naturally occurring isotope of gold, 197Au, the atomic
weight of gold given in the modern periodic table is 196.96655, but not 197.
Explain.
197 is the mass number of gold-197 isotope (the only isotope found in nature); it is an
exact number and an integer as it represents the sum of the numbers of protons and
neutrons present in each atom of gold. 196.96655 amu is the mass of gold atoms
relative to the mass of carbon-12 atoms with the exact mass of 12 amu. Carbon-12
atom is the modern standard for the atomic mass scale; there no other atom with the
mass expressed as an exact number. The only way 197 and not 196.96655 would
appear in the periodic table below the Au symbol is to make the standard by
assigning exactly 197 amu to one gold-197 atom.
5. (4 points) Gold is the most malleable of all metals; a single gram of it can be beaten into a
sheet of one square meter. What would be the thickness of such sheet of gold in
millimeters?
Show all the steps of your work. Each measured or calculated number should have a unit.
The final answer should be recorded with the proper number of significant digits.
Volume = Mass / Density = 1.00 g / (19.3 g/cm3) = 0.0518 cm3
Volume = Area × Thickness = 1.00×104 cm2 × Thickness = 0.0518 cm3
Thickness = 0.0518 cm3 / (1.00×104 cm2) = 5.18×10−6 cm = 5.18×10−5 mm =
= 0.0000518 mm