Chemistry · Ch 1 — Some Basic Concepts of Chemistry
Formula Mass
Formula Mass
Some substances — sodium chloride is the standard example — do not actually exist as discrete, separate molecules at all. Instead, in a sodium chloride crystal, the sodium (cationic) and chloride (anionic) ions are arranged throughout a continuous, repeating three-dimensional lattice, where each sodium ion (Na+) is surrounded by six chloride ions (Cl-) at an equal distance, and vice versa for each chloride ion. Because there is no single, isolated 'molecule' of NaCl to point to, the formula 'NaCl' is used purely as a REPRESENTATIVE formula unit for sodium chloride rather than as an actual molecule, and correspondingly the term 'molecular mass' is not strictly appropriate for such ionic compounds — chemists instead use the term 'formula mass'. The formula mass of a substance is defined as the sum of the atomic masses of all the atoms present in its (represent …
Worked out. Problem: find the formula mass of (i) NaCl and (ii) Cu(NO3)2. Solution (own words): (i) NaCl's formula mass is the average atomic mass of sodium (23.0 u) plus the average atomic mass of chlorine (35.5 u), giving 58.5 u. (ii) Cu(NO3)2's formula mass is the average atomic mass of copper (63.5 u) plus 2 times [the average atomic mass of nitrogen (14 u) plus 3 times the average atomic mass of oxygen (16 u)], i.e. 63.5 + 2 x (14 + 48) = 63.5 + 124 = 187.5 u. This shows how a formula mass for an ionic compound is worked out by summing the atomic masses of every atom shown in its formula, e …