Chemistry Reference
In-Depth Information
7.111 Calculate the molecular formula of a substance if its
percent composition is 85.63% C and 14.37% H, and its
molar mass is approximately 85 g/mol.
7.112 Calculate the molecular formula of a compound if a
sample contains 135 g of phosphorus and 175 g of oxy-
gen and its molar mass is 284.
7.113 Octane and heptane are two ingredients of gasoline.
Octane has 84.12% carbon and 15.88% hydrogen, and
heptane has 83.90% carbon and 16.10% hydrogen.
Their molecular masses are 114 amu and 100 amu,
respectively. What are their molecular formulas?
General Problems
7.114
A 6.09-mg sample of a hydrocarbon was burned in air.
The products were 17.9 mg of and 11.0 mg of
(a) What mass of oxygen was used in the combustion?
(b) How many millimoles of carbon was present in the
products, and in the sample?
(c) How many millimoles of hydrogen was present in
the products and in the sample?
(d) What is the empirical formula of the sample?
7.115 A 17.1-mg sample of a hydrocarbon was burned in air.
The products were 47.0 mg of and 38.4 mg of wa-
ter. What is the empirical formula of the hydrocarbon?
7.116
A certain carbohydrate (a compound containing carbon
plus hydrogen and oxygen in a 2:1 atom ratio) is 40.0%
carbon. Calculate its empirical formula.
7.117 Calculate the percent composition of soluble saccharin,
7.126 Calculate the number of molecules of vitamin A,
in 2.34 g of vitamin A.
7.127
(a) Vitamin has one cobalt atom per formula unit.
The compound is 4.348% Co. Calculate its molar
mass.
(b) Vitamin has two oxygen atoms per formula unit.
The compound is 4.03% O. Calculate its molar
mass.
7.128
A sample is 39.2% KCl by mass, and the rest is water.
Calculate the number of molecules of water in 14.4 g of
the sample.
CO
2
H
2
O.
C
20
H
30
O,
B
12
D
1
CO
2
7.129 A 6.055-g sample of a hydrate of copper(II) sulfate,
is heated until all the water is driven off.
After the anhydrous salt cools, its mass is 3.870 g.
Calculate the value of
x.
(
Hint:
Treat this problem as an
empirical formula problem with one of the “elements”
CuSO
4
#
x
H
2
O,
C
7
H
4
NNaO
3
S.
H
2
O.)
7.118
How many moles of carbon atoms is present in the
quantity of that contains 4.14 g of hydrogen?
7.119 Calculate the percent composition of ammonium
cyanide.
7.120 Calculate the number of hydrogen atoms in 42.7 g of
a compound that contains 91.25% carbon and 8.75%
hydrogen.
7.121 Calculate the number of molecules in 2.79 g of a com-
pound that has a molar mass of 92.13 g/mol and con-
tains 91.25% carbon and 8.75% hydrogen.
7.122 Calculate the number of hydrogen atoms in 12.8 g of
a compound whose percent composition is 15.88% H
and 84.12% C.
7.123 Calculate the number of hydrogen atoms in 41.8 g of a
compound that contains 4.14% hydrogen.
7.124 Calculate the number of molecules in 31.7 g of a com-
pound that has a molar mass of 82.2 g/mol.
7.125 Calculate the number of carbon atoms in 1.000 gallon
of octane, a major component of gasoline
(1 gallon
3.785 L; density
0.7025 g/mL).
C
2
H
6
O
7.130 A scientist isolates a pure substance from a newly dis-
covered plant in the Amazon River basin. What data
does the scientist need to start to determine whether the
substance is a new compound and what its formula is?
7.131 Calculate the percent error in rounding off the atomic
mass of each of the following elements to three signifi-
cant digits:
(a) Hydrogen (b) Sulfur
(c) Lead (d) Neon
(e) Carbon (f) Fluorine
7.132
A different method may be used to calculate the molec-
ular formula from percent composition data plus a mo-
lar mass: First, calculate the mass of each element in
1.00 mol of compound. Next, calculate the number of
moles of each of the elements in the mole of compound.
Those results yield the molecular formula. Use this
method to calculate the molecular formula of a hydro-
carbon (a compound of carbon and hydrogen only) that
contains 87.73% C and has a molar mass of 82.0 g/mol.
C
8
H
18
,
