Name
Date
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Chemistry – Unit 4 Worksheet 3
Use the following information about the masses of elements in each pair of
compounds to help you suggest formulas that account for these ratios.
1. Compounds of carbon and oxygen
Compound A: 57.1 g O / 42.9 g C
Compound B: 72.7 g O and 27.3 C
a. Determine the value of the ratio
mass O
in each compound. A 1.33 B 2.67
mass C
b. How does the mass ratio for compound B compare to that in compound A?
The ratio in B is twice as great as that in A.

c. Express these ratios as improper fractions.
A = 4/3
B = 8/3
d. For each hypothesis, sketch particle diagrams for the compounds of A and B that
account for these mass ratios. Write the formula for the compound in each
diagram.
Hypothesis 1
Atoms of C and O have the same mass
A
Hypothesis 2
Atoms of O are heavier than C atoms by
the ratio in compound A.
A
CO
C3O4
B
B
CO2
C3O8
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2. Compounds of copper and oxygen
Compound A: 79.9 g Cu / 20.1 g O
Compound B: 88.8 g Cu / 11.2 g O
a. Determine the value of the ratio
mass Cu
in each compound. A 3.98 B 7.93
mass O
b. How does the mass ratio for compound B compare to that in compound A?
The mass ratio for compound B is twice as large as that in compound A

c. Express these ratios as improper fractions.
A = 4/1
B = 8/1
d. For each hypothesis, sketch particle diagrams for the compounds of A and B that
account for these mass ratios. Write the formula for the compound in each
diagram.
Hypothesis 1
Atoms of Cu and O have the same mass
A
Hypothesis 2
Cu atoms are heavier than O atoms by the
ratio in compound A.
A
CuO
Cu4O
B
B
Cu2O
Cu8O
Which hypothesis seems more reasonable to you? Justify your answer.
Hypothesis 2 is more reasonable for two reasons:
1) The compounds in Hypothesis 2 are simpler than those in Hypothesis 1.
2) It’s not reasonable that all atoms have the same mass.
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Use the hypothesis you have chosen to suggest formulas for the following pairs of
compounds.
3. Compounds of copper and chlorine
Compound A: 35.9 g of Cl / 64.1 g of Cu
Compound B: 52.8 g of Cl / 47.2 g Cu
a. Determine the value of the ratio
mass Cl
in each compound. A 0.560 B 1.12
mass Cu
b. How does the mass ratio for compound B compare to that in compound A?
The mass ratio in compound B is twice as great as that in compound A.

c. What are the simplest formulas for compounds A and B? Explain your
reasoning.
Students are likely to write ClCu and Cl2Cu. You can suggest that it is
customary to put the metal element first in the formula of the compound.
4. Compounds of iron and chlorine (be careful!)
Compound A: 56.0 g of Cl / 44.0 g of Fe
Compound B: 65.6 g of Cl / 34.4 g of Fe
a. Determine the value of the ratio
mass Cl
in each compound. A 1.27 B 1.91
mass Fe
b. The ratios you determined in step (a) give the mass of Cl that combines with 1 g

of Fe in each compound. To determine how the mass of Cl in compound B
compares to the mass of Cl in compound A for the same amount of Fe, divide
these ratios and express the answer as an improper fraction. What does this
fraction tell you about the number of Cl atoms in each of the two compounds?
1.91
3
 1.50 
This suggests that there are 3 Cl atoms for every Fe in
1.27
2
compound B and 2 Cl atoms for every Fe in compound A.

c. What would be the formulas of the two compounds, assuming that each
compound contains one atom of Fe?
Compound A is FeCl2
Compound B is FeCl3
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