Empirical Versus Molecular Formulas

8/11/2019 Empirical Versus Molecular Formulas

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Empirical versus Molecular Formulas

The formulas we have calculated in the preceding section express the simplest atomic

ratio between the elements in the compound. Such formulas are called empirical

formulas. An empirical formula does not necessarily represent the actual numbers ofatoms present in a molecule of a compound; it represents only the ratio between those

numbers. The actual numbers of atoms of each element that occur in the smallest

freely existing unit or molecule of the compound is expressed by the molecular

formula of the compound. The molecular formula of a compound may be the

empirical formula, or it may be a multiple of the empirical formula. For example, the

molecular formula of butene, C4H8, shows that each freely existing molecule of

butene contains four atoms of carbon and eight atoms of hydrogen. Its empirical

formula is CH2. One molecule of ethylene (molecular formula C2H4) contains two

atoms of carbon and four atoms of hydrogen. Its empirical formula is CH2. Both have

the same empirical formula, yet they are different compounds with different molecularformulas. Butene is C4H8, or four times the empirical formula; ethylene is C2H4, or

twice the empirical formula.

Table 6.7 shows three groups of compounds. Within each group, the compounds have

the same empirical formula and percent composition but different molecular formulas.

That they are different compounds is shown by their different boiling points.

TABLE 6.7Compounds with the same empirical formula but different molecular formulas

Empirical formula Compound

Molecular

formula

Boiling

point, C

CH (92.2% C; 7.8% H)acetylene C2H2 -84

benzene C6H6 80

CH2(85.6% C; 14.4% H)

ethylene C2H4 -103

butene C4H8 -6.3

cyclohexane C6H12 80.7

CH2O (40.0% C; 6.7% H;

53.3% O)

formaldehyde CH2O -21

acetic acid C2H4O 117

glyceraldehyde C3H6O3 140

The molecular formula of a compound can be determined from the empirical formula

if the formula weight, or molecular weight, is known.


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Example

The empirical formula of hexane is C3H7. Itsmolecular weight is 86.2 amu. What is the molecular

formula of hexane?

Solution

The molecular formula of a compound is a multiple of

its empirical formula. the molecular formula weight is

some multiple of the empirical formula weight. We

know the empirical formula dn thus can calculate theempirical formula weight. We can calculate what

multiple the molecular formula weight is of theempirical formula weight. As stated, the molecularformula is the same multiple of the empirical formula.

1. Calculate the formula weight of C3H7.

2. Calculate the ratio between the molecular weight

and the empirical weight:

3.The molecular formula must be twice the emp;irical

formula:

(C3H7)2or C6H14

Two kinds of data are needed to determine the molecular formula of a compound: (1)

its composition, from which we can calculate its empirical formula, and (2) its

molecular weight. The molecular weight will be a multiple of the empirical formula

weight. The molecular formula is the same multiple of the empirical formula.


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Example

The compound ethylene glycol is often used as an antifreeze. Itcontains 38.7% carbon, 9.75% hydrogen, and the rest oxygen. The

molecular weight of ethylene glycol is 62.07 g. What is the molecularformula of ethylene glycol?

Solution

1.Calculate the empirical formula. Assume 100 g of the compound,

which will contain 38.70 g carbon, 9.75 g hydrogen and the rest

oxygen

? g O = 100 g - 38.70 g C - 9.75 g H = 51.55 g O.

2. Calculate the moles of each element present:

3.Next calculate the ratio of molecular weight to empircal formula

weight. The molecular weight is given. The empirical formula is

CH3O, so the empirical formula weight is 12.01 + 3(1.008) + 16.00 =31.03.

Therefore the molecular formula is twice the empirical formula:

C2H6O2.


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ExampleThe compound dioxane contains only carbon, hydrogen, and oxygen. When 0.956 gdioxane is burned, 1.91 g carbon dioxide and 0.782 g water are formed. In another

experiment, it was determined that 6.04x10-3 mol dioxane weighs 0.532 g. What is the

molecular formula of dioxane?

Solution

1.Calculate the mass of carbon, hydrogen, and oxygen in 0.956 g dioxane.

2. Using the data from step 1 above, calculate the empirical formula of dioxane.

The empirical formula is:

C0.043H0.087O0.022

3.Calculate the molecular weight of dioxane.


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4. Calculate the molecular formula of dioxane.

The empirical formula weight is 2(12.0) + 4(1.01) + 16.0 = 44.0. The molecular weight is

88.08. The ratio of molecular weight to empirical weight is:

Thus the molecular formula is: C4H8O2.

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Empirical Versus Molecular Formulas