Many chemists use the terms formula mass and molecular mass interchangeably when dealing with chemical compounds of known formula. But there is a slight difference between the two terms, as explained below. The formula mass of a compound is the sum of the atomic masses of all the atoms in a formula unit of the compound, whether it is ionic or molecular (covalent). The formula mass is based on the ratio of different elements in a formula, as opposed to the molecular mass, which depends on the actual number of each kind of atom (compare section 6.2, “Empirical Formula”). Formula masses are relative since they are derived from relative atomic masses. For example, the formula mass of phosphoric acid, H3PO4, is 97.98 atomic mass units (amu), which is obtained by adding the atomic masses (taken from the periodic table) of the elements in one formula unit (i.e., 3 H + 1 P + 4 O). (3×At. wt. of H)+(1×At. wt. of P)+(4×At. Wt. of O) = (3×1.00)+(1×30.97)+(4×16.00) = 97.97 Once the actual formula of a chemical substance is known, the molecular mass can be determined in a manner similar to calculating the formula mass. The molecular mass of a compound is the sum of the atomic masses of all the atoms in one molecule of the compound. The term applies only to compounds that exist as molecules, such as H2O, SO2, and glucose, C6H12O6. For example, the molecular mass of ethanol, C2H5OH, is: (2×C)+(6×H)+(1×O) (2×12.0)+(6×1.0)+(1×16.0) = 46 When ionic compounds such as NaCl, Zn(NO3)2, andNH4Cl, are in the crystalline state or in solution form, they do not contain physically distinct uncharged molecular entities. Therefore chemists often use the term formula mass to represent the total composition of such substances. Example 4.1 Calculate the formula mass (FM) of NaOH using a table of atomic masses (AM).
On the atomic masses (weights?) Of the elements
