06/28/16
CHEM 1A Quiz 2 (30 points)
ANSWER KEY
Name _________________________________
1. (6 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
found in nature.
atomic weight
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).
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.
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
A compound made of atoms of two elements.
binary compound
2. (3 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 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.
PLEASE TURN OVER!!!
3. (3 points) On the old atomic mass scale used by physicists, the mass of oxygen-16 atom was
assigned to be exactly 16 amu. What would be the atomic weight of tin on that scale? Show work.
mass of the “avg.” Sn atom on the old scale
mass of 16O atom on the old scale
X amu
16 amu (exactly)
=
118.710 amu
15.9949 amu
=
mass of the “avg.” Sn atom on the new scale
mass of 16O atom on the new scale
A.W. of Sn on the old scale = 118.748 amu
4. (6 points)
(a) How many different CO (carbon monoxide) molecules can possibly be found in nature?
Explain.
Six. There are 2 natural isotopes of C and 3 natural isotopes of O. That gives 2×3 = 6
combination one C atom bonded with one O atom.
(b) Which is the most abundant of the possible CO molecules? Which is the second most
abundant? Explain.
12 16
C O is the most abundant; 13C16O is the second most abundant. They will have the highest
fractional abundances: 0.9893×0.99757 = 0.9869 (about 99%) and 0.0107×0.99757 =
0.010697 (about 1%) respectively (all other combinations will be less abundant).
(c) What is the average mass of the possible CO molecules? Explain.
A.W. of C = 12.0107 amu; A.W. of O = 15.9994 amu
M.W. of CO = 12.0107 amu + 15.9994 amu = 28. 0101 amu
5. (6 points) The single proton that forms the nucleus of the hydrogen atom has a radius of
approximately 1.0×10−13 cm. The hydrogen atom itself has a radius of approximately 52.9 pm.
What fraction of the space within the atom is occupied by the nucleus? (Hint: The volume of the
sphere is proportional to its radius cubed.) Show work.
(1.0×10−13 cm)3
(52.9×10−10 cm)3
=
6.8×10−15
6. (6 points) A piece of capillary tubing was calibrated in the following manner. A clean sample of
the tubing weighed 3.247 g. A thread of mercury, drawn into the tubing, occupied a length of
23.75 mm, as observed under the microscope. The weight of the tubing with the mercury was
3.489 g. Assuming that capillary bore is a uniform cylinder, find the diameter of the bore. Show
work.