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100 Chemistry: Matter and Change
Solving Problems: A Chemistry Handbook
SOLVING PROBLEMS:
A CHEMISTRY HANDBOOK
CHAPTER 10
Molecular formulas For many compounds, the empirical formula is not the true formula. For example, you found the empirical
formula for acetic acid in Practice Problem 26 to be CH 2 O.
Chemists have learned, though, that acetic acid is a molecule with
the formula C 2 H 4 O 2 , which is the molecular formula for acetic acid.
A molecular formula tells the exact number of atoms of each element in a molecule or formula unit of a compound. Notice that the
molecular formula for acetic acid (C 2 H 4 O 2 ) has exactly twice as
many atoms of each element as the empirical formula (CH 2 O). The
molecular formula for a compound is always a whole-number multiple of the empirical formula.
In order to determine the molecular formula for an unknown
compound, you must know the molar mass of the compound in addition to its empirical formula. Then you can compare the molar mass
of the compound with the molar mass represented by the empirical
formula as shown in the following example problem.
Example Problem 10-9
Determining a Molecular Formula
Maleic acid is a compound that is widely used in the plastics and textiles industries. The composition of maleic acid is 41.39% carbon,
3.47% hydrogen, and 55.14% oxygen. Its molar mass is 116.1 g/mol.
Calculate the molecular formula for maleic acid.
Start by determining the empirical formula for the compound as
shown in Example Problem 11-8.
moles of C (in 100 g) ϭ 41.39 g C ϫ
ϭ3.446 mol C
moles of H (in 100 g) ϭ 3.47 g H ϫ
ϭ3.442 mol H
moles of O (in 100 g) ϭ 55.14 g O ϫ
ϭ3.446 mol O
The numbers of moles of C, H, and O are nearly equal, so it is not
necessary to divide through by the smallest value. You can see by
inspection that the smallest whole-number ratio is 1C : 1H : 1O, and
the empirical formula is CHO.
1 mol O
ᎏᎏ
15.999 g O
1 mol H
ᎏᎏ
1.008 g H
1 mol C
ᎏᎏ
12.011 g C
▲
100 Chemistry: Matter and Change
Solving Problems: A Chemistry Handbook
SOLVING PROBLEMS:
A CHEMISTRY HANDBOOK
CHAPTER 10
Molecular formulas For many compounds, the empirical formula is not the true formula. For example, you found the empirical
formula for acetic acid in Practice Problem 26 to be CH 2 O.
Chemists have learned, though, that acetic acid is a molecule with
the formula C 2 H 4 O 2 , which is the molecular formula for acetic acid.
A molecular formula tells the exact number of atoms of each element in a molecule or formula unit of a compound. Notice that the
molecular formula for acetic acid (C 2 H 4 O 2 ) has exactly twice as
many atoms of each element as the empirical formula (CH 2 O). The
molecular formula for a compound is always a whole-number multiple of the empirical formula.
In order to determine the molecular formula for an unknown
compound, you must know the molar mass of the compound in addition to its empirical formula. Then you can compare the molar mass
of the compound with the molar mass represented by the empirical
formula as shown in the following example problem.
Example Problem 10-9
Determining a Molecular Formula
Maleic acid is a compound that is widely used in the plastics and textiles industries. The composition of maleic acid is 41.39% carbon,
3.47% hydrogen, and 55.14% oxygen. Its molar mass is 116.1 g/mol.
Calculate the molecular formula for maleic acid.
Start by determining the empirical formula for the compound as
shown in Example Problem 11-8.
moles of C (in 100 g) ϭ 41.39 g C ϫ
ϭ3.446 mol C
moles of H (in 100 g) ϭ 3.47 g H ϫ
ϭ3.442 mol H
moles of O (in 100 g) ϭ 55.14 g O ϫ
ϭ3.446 mol O
The numbers of moles of C, H, and O are nearly equal, so it is not
necessary to divide through by the smallest value. You can see by
inspection that the smallest whole-number ratio is 1C : 1H : 1O, and
the empirical formula is CHO.
1 mol O
ᎏᎏ
15.999 g O
1 mol H
ᎏᎏ
1.008 g H
1 mol C
ᎏᎏ
12.011 g C
▲