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 of atoms 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 molecular formulas. 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.
|CH (92.2% C; 7.8% H)||acetylene||C2H2||-84|
|CH2 (85.6% C; 14.4% H)||ethylene||C2H4||-103|
|CH2O (40.0% C; 6.7% H;
The molecular formula of a compound can be determined from the empirical formula if the formula weight, or molecular weight, is known.
The empirical formula of hexane is C3H7. Its molecular weight is 86.2 amu. What is the molecular formula of hexane?
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 the empirical formula weight. We can calculate what multiple the molecular formula weight is of the empirical formula weight. As stated, the molecular formula 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)2 or 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.
The compound ethylene glycol is often used as an antifreeze. It contains 38.7% carbon, 9.75% hydrogen, and the rest oxygen. The molecular weight of ethylene glycol is 62.07 g. What is the molecular formula of ethylene glycol?
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.
The compound dioxane contains only carbon, hydrogen, and oxygen. When 0.956 g dioxane 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?
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.
3. Calculate the molecular weight of dioxane.
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.