The carbon powder formula gives the molecular formula for a compound that contains only atoms of carbon. For organic compounds, the atoms are written first, followed by hydrogen atoms, and then the other elements in alphabetical order, as is the case for methyl alcohol (CH4O).
For inorganic compounds, the atoms of each element are listed starting from the one farthest to the left in the periodic table, as in CO2 or SF6. Fluorine and chlorine are in the same group, so they are listed beginning with the lower element and working up, as in CClF. Then subscripts are added to give the molecular formula.
Butane, an important gas used in cooking and as a fuel, has the empirical formula C2H5. However, it contains two formula units, so its molecular formula is C4H10.
Sucrose, a common sweetener, has 12 carbon atoms, 11 oxygen atoms, and 22 hydrogen atoms. It has the empirical formula CHO.
Nitrous oxide, an anesthetic, has 2 nitrogen atoms and 1 oxygen atom per molecule. It has the empirical formula HO2CH2.
Freon-11, a propellant used to power jet airplanes, has carbon, chlorine, and fluorine in its molecular formula. It can be viewed as either an inorganic compound or an organic compound, since fluorine replaced hydrogen in the original composition.
When writing the empirical formula, the cation must be balanced by the anions, with the overall charge on the ions being equal to or less than the total charge on the ions in the formula unit. The same rule applies to polyatomic ions, as is illustrated in Table 3.1.4 “Common Polyatomic Ions and Their Names.”