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Name: ___KEY______________________
Section: _____________
Chapter 2 Homeowrk:
Atoms, Molecules, and Ions
1.
Briefly define the following:
a. Law of Conservation of Mass
Matter is neither created nor destroyed in a chemical reaction – molecules change (atoms
rearrange) to create new substances.
b. Law of Definite Proportions
All samples of a given substance will have the same ratio of atoms by mass (e.g., carbon
dioxide is always CO2).
c. Dalton’s Atomic Theory
4 postulates: all matter is made of atoms (once thought to be indivisible); all atoms of a
given elements are identical (atoms of different elements are different); atoms are not
created or destroyed in a reaction; atoms combine in small, whole-number ratios to form
compounds.
d. Law of Multiple Proportions
Two different compounds made up of the same elements will have mass ratios that are
small whole-number ratios of each other. For example, ratio of oxygen in CO to oxygen in
CO2 is 1:2.
2.
Describe the contribution made by the following scientists:
a. Priestley
Isolated oxygen by heating HgO in a closed container to observe conservation of mass.
b. Thomson
Worked with cathode ray tubes to discover the presence of electrons in the atom.
c. Millikan
Used the oil drop experiment to calculate the charge (in Coulombs) of an electron.
d. Rutherford
Used the gold foil experiment to determine the structure of an atom (relative positions of
protons and electrons).
3. Fill in the table below:
Symbol
Atomic Number
1
Mass Number
Number of Neutrons
1
1
0
O
8
17
9
Hg
80
199
119
Cu
29
63
34
35
71
36
1H
17
199
63
71
Br
4. How many gold-197 atoms are in a 0.156 gram sample? (Assume that the atomic mass of gold197 is very close to 197).
Chapter 2 Worksheet
page 1
Name: ___KEY______________________
Section: _____________
4.77 x 1020 atoms
5. Classify the following as a mixture or pure substance, and atom, molecule, or compound.
a
b
c
d
Pure substance;
Molecules
Mixture of atoms and
compounds
Pure substance;
compound
Pure substance;
Atoms
e
f
g
h
Mixture of atoms and
molecules
Mixture of atoms and
molecules
Mixture of molecules
Mixture of atoms
i
j
k
l
Mixture of compounds
Pure substance;
Atoms
Mixture of atoms and
molecules
Pure substance;
Molecules
6. Describe the difference between ionic and covalent compounds.
Ionic bonds form when electrons transfer from one atom to another (between metals and
nonmetals). Covalent bonds form when electrons are shared between atoms (between two or more
nonmetals).
7.
CH4
Circle the substances that are ionic in the list below:
LiCl
C2H4
NaNO3
CoCl2
CO2
KBr
8. Calculate the atomic weight for the element lithium. Lithium-6 has an natural percent
abundance of 7.5% and an atomic mass of 6.015122 amu. Lithium-7 has a natural abundance
of 92.5% and an atomic mass of 7.016003 amu.
Chapter 2 Worksheet
page 2
Name: ___KEY______________________
Section: _____________
6.94amu
9.
What is the difference between atomic mass and molar mass? Molecular mass and molar
mass?
Atomic mass refers to the mass of 1 atom, while molar mass refers to the mass of 6.022x1023
atoms. Molecular mass refers to the mass of 1 molecule, while molar mass refers to the mass
of 6.022x1023 molecules. On both cases, the difference is 1 mol (Avogadro’s number).
10. If a C-12 atom has an atomic mass of 12 amu (exactly) and a molar mass of 12 g (exactly),
how many grams are in 1 amu?
1.6605x10-24g
11. What is the difference between an element’s atomic mass number and atomic mass?
An element’s atomic mass number is a whole number that denotes the number of protons and
the number of neutrons. Atomic mass is not a whole number (except for Carbon-12). It is not
a whole number as it usually the average of all naturally occurring isotopes. And technically,
even a specific isotope’s atomic mass is not a whole number (for numerous reasons. For
example, the mass of a proton and a neutron are not exactly the same. But this is beyond the
scope of this class).
12. Which has a higher number of atoms, a 20.00g sample of carbon atoms or a 20.00g sample of
N atoms.
A 20g sample of carbon atoms. C ~ 1x1024 atoms
N ~ 9x1023 atoms
13. How many atoms of He has the same mass as 1.00x1012 carbon atoms? How many atoms of
Al has this same mass?
3.00x1012 He atoms (this makes sense since the mass of C ~ 3x the mass of He)
4.45x1011 Al atoms
Chapter 2 Worksheet
page 3