Chemical formulas such as HClO_{4} can be divided into empirical formula, molecular formula, and structural formula. Chemical symbols of elements in the chemical formula represent the elements present, and subscript numbers represent mole proportions of the proceeding elements. Note that no subscript number means a subscript of 1.
From a chemical point of view, an element contained in the substance is a fundamental question, and we represent the elemental composition by a chemical formula, such as H_{2}O for water. This formula implies that the water molecules consist of 2 hydrogen, and 1 oxygen atoms. The formula H_{2}O is also the molecular formula of water. For non-molecular substances such as table salt, we represent the composition with an empirical formula. Sodium chloride is represented by NaCl, meaning that sodium and chlorine ratio in sodium chloride is 1 to 1. Again, the subscript 1 is omitted. Since table salt is an ionic compound, the formula implies that numbers of Na^{+} ions, and Cl^{-} ions are the same in the solid. The subscript numbers in an empirical formula should have no common divisor.
H H | | H-C-C-O-H | | H H |
Structural of CH_{3}CH_{2}OH |
A 3-Dimensional structure of C_{6}H_{12} |
You may refer to a substance by its name, and recognize it by its properties. Properties are related to the structure and the composition of the molecules. Knowing the chemical formula is a giant step towards understanding a substance.
The formula weight is the sum of all the atomic weights in a formula. The evaluation of formula weight is illustrated in this example.
Example 1
Solution:
The formula also indicates a mass as the sum of masses calculate this way
2*1.008 + 32.0 + 4*16.0 = 98.0
where 1.008, 32.0 and 16.0 are the atomic weights of H, S, and O respectively.
Discussion:
If the formula is a molecular formula, the mass associated with it is called
molecular mass or molecular weight. As an exercise,
work out the following problem.
What is the molecular weight of caffeine, C_{8}H_{10}N_{4}O_{2}?
The diagram shown here is a model of the caffeine molecule.
With the aid of a table of atomic weights, a formula indirectly represents the formula weight. If the formula is a molecular formula, it indirectly represents the molecular weight. For simplicity, we may call these weights molar masses, which can be formula weights or molecular weights.
A chemical formula not only represents what a substance is made of, it provides a great deal of information about the substance. Do you know that chemical formulas are used all over the world, regardless of the language? Chinese, Russian, Japanese, African, and South Americans use the same notations we do. Thus, H_{2}S is recognized as a smelly gas all over the world. Chemical formula is an international or universal language.
Example 2
Solution:
From example 1, we know that there are 32.0 g or S in 98.0 g or sulfuric
acid. Thus the weight percentage is
Weight percentage = 32/98 = 32.7%
From the formula, there is one S atom among 7 atoms in
H_{2}SO_{4}
Mole percentage = 1/7 = 14.3%
Discussion:
You have learned what weight and mole percentages are and how to evaluate
them in this example. As an exercise, work out the the following problem:
What are the weight and mole percentages of C, H, N, and O for caffeine, C_{8}H_{10}N_{4}O_{2}?
If you are a researcher and you made a new compound that no one has ever made it before, then you need to determine its empirical or molecular formula. For an organic compound, you burn it completely to convert all carbon (C) to CO_{2}, and all hydrogen (H) to H_{2}O.
Nitrogen is determined by converting it to NH_{3}. The amount of NH_{3} can be determined by titration, and the percentage can also be determined.
Percentage of O is usually obtained by subtracting all percentages of C, H, and N, if the compound does not contain any other element.
Example 3
Solution:
Assume that you have 100 g of the compound, then you have 92.3 g of carbon
and 7.7 g of hydrogen. Thus the mole ratio of C to H should be
92.3 7.7 ---- : ----- = 7.7 : 7.7 = 1 : 1 12 1.008Thus, the empirical formula is CH.
Discussion:
You have learned how to determine a chemical formula if the percentages of
various elements present in the compound are known in this example. To test
your skill, you may be asked to work out the empirical formula of any
compound. Try this problem:
Aspartic acid contains 36.09% C, 5.30% H, 10.52% N, and 48.08 O by weight.
What is the empirical formula for aspartic acid?
Aspartic acid is one of the non-essential aminoacids, usually present in young
plants. It is obtained by hroolysis of asparagine, which is abundant in
asparagus.
Example 4
Solution:
The formula weight of CH is 13.0.
Since 78/13 = 6,
the molecular formula is C_{6}H_{6}, the formula for benzene.
Discussion:
This example illustrates the difference between empirical and molecular
formula, for which, the molecular weight must be known.
Example 5
Hint:
1 mol C 1 mol CO_{2} 44.0 g CO_{2} 1 g CH --------- ----------- ------------ = 3.38 g CO_{2} 13 g CH 1 mol C 1 mol CO_{2}Use the same method to calculate the amount of H_{2}O produced (Ans. 0.692 g).
Example 6
Hint:
Amounts of carbon and H in 3.14 g of carbon dioxide,
12 g C 1 mol C 3.14 g CO_{2} -------- --------- = 0.0714 mol C 44 g CO_{2} 12 g CThus, mole ratio of C : H is 0.0714 : 0.143 = 1 : 2. Therefore, the empirical formula is CH_{2}
2 g H 1 mol H 1.29 g H_{2}O -------- --------- = 0.143 mol H 18 g H_{2}O 1 g H
Example 7
Hint:
You should understand the reason for using this formula to calculate it:
3*35.453 ---------------------------- = 89.094% (weight percentage) 3*35.453 + 12.011 + 1.00794Discussion:
© CChieh@UWaterloo.ca