Chemical Formulas
Chemical formulas such as HClO4 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 H2O for water. This formula implies that the water molecules consist of 2 hydrogen, and 1 oxygen atoms. The formula H2O 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.
A structural formula reflects the bonding of atoms in a molecule or ion. For example, ethanol can be represented by CH3CH2OH. This is a simple way of representing a more elaborated structure shown on your left. Molecular structures are often beautiful, but the representation is an artwork. For example, a 3-dimensional structure of cyclohexane is shown on the right. This is a chair form, and another structure has a boat form. You will learn more about it in organic chemistry. The molecular formula of benzene is C6H6, and its empirical formula is CH.
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.
Formula Weights, Molecular Weights and Molar Masses
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
What is the formula weight of sufuric acid H2SO4?
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, C8H10N4O2?
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, H2S is recognized as a smelly gas all over the world. Chemical formula is an international or universal language.
Weight percentage and mole percentage
A chemical formula not only gives the formula weight, it accurately represents the percentages of elements in a compound. On the other hand, if you know the percentage of a compound, you may figure out its formula. Percentage based on weights is called weight percentage, and percentage based on the numbers of atoms or moles is called mole percentage.
Example 2
What are the weight and mole percentages of S in sufuric acid?
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 H2SO4
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, C8H10N4O2?
Determination of Chemical Formulas
How would you find the chemical formula of a substance? If you know the substance, its formula and other information is usually listed in a handbook. Handbooks such as the CRC Handbook of Chemistry and Physics contain information on millions of substances.
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 CO2, and all hydrogen (H) to H2O.
CxH2y =(burned in O2)=> x CO2 + y H2O
Thus, from the weight of CO2 and H2O produced by burning a definite amount of the substance, you can figure out the percent of C and H in the compound.
Nitrogen is determined by converting it to NH3. The amount of NH3 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
A compound containing 92.3 weight percent of carbon and 7.7 weight percent of H. What is the empirical formula?
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 1.008
Thus, 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
A compound with an empirical formula of CH has a molecular weight of 78 g/mol. What is the molecular formula?
Solution:
The formula weight of CH is 13.0.
Since 78/13 = 6,
the molecular formula is C6H6, 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
When 1.00 g of benzene is burned, how much CO2 and H2O should be produced?
Hint:
1 mol C 1 mol CO2 44.0 g CO2
1 g CH --------- ----------- ------------ = 3.38 g CO2
13 g CH 1 mol C 1 mol CO2
Use the same method to calculate the amount of H2O produced (Ans. 0.692 g).
Example 6
When 1.00 g of a compound containing only carbon and hydrogen is burned completely, 3.14 g of CO2 and 1.29 g of H2O is produced. What is the empirical formula?
Hint:
Amounts of carbon and H in 3.14 g of carbon dioxide,
12 g C 1 mol C
3.14 g CO2 -------- --------- = 0.0714 mol C
44 g CO2 12 g C
2 g H 1 mol H
1.29 g H2O -------- --------- = 0.143 mol H
18 g H2O 1 g H
Thus, mole ratio of C : H is 0.0714 : 0.143 = 1 : 2. Therefore, the empirical formula is CH2
Discussion:
The molecular formula for ethylene is C2H4 and cyclohexane is C6H12. What are their molecular weights?
Example 7
Chloroform is a common solvent used in chemical labs. It has a molecular formula of CHCl3. What is the weight percentage of chlorine (Cl)? (Atomic weight, Cl, 35.453; H, 1.00794; C, 12.0110)
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 + 1.00794
Discussion:
What is the weight percentage of C in CCl4?
regards
faryal shah