combination of one sodium ion and one chloride ion as a formula unit. The formula
unit of an ionic compound is the smallest electrically neutral collection of ions. The
ratio of atoms in the formula unit is essentially the same as in the chemical formula.
Since a formula unit is only a fragment of a vast network of ion (crystal), a formula
unit cannot be distinguished as distinct. Therefore, it is incorrect to label a formula
unit as a molecule.
Ions such as Na+, Mg2+ and Cl- are monatomic, meaning that each consists of a
single ionized atom. Alternately, and ion made up of two or more atoms are called
polyatomic ions (e.g. NO3-). (Study polyatomic ions)
The Mole Concept and Chemical Compounds
Formula mass: the mass of a formula unit in atomic mass units. It is appropriate to
use the term formula mass, but for molecular compounds, the formula unit is
actually a molecule, so the term molecular mass is used. The molecular mass is
the mass of a molecule in atomic mass units (amu).
Mole of a Compound
The molar mass is the mass of one mole of compound – one mole of molecules of a
molecular compound and one mole of formula units of an ionic compound.
Problem: An analytical balance can detect a mass of 0.1 mg. What is the total
number of ions present in this minimally detectable quantity of MgCl2?
Solution: Use molar mass to convert from mass to number of moles of MgCl2. Then
use Avogadro constant as a conversion factor to convert moles to number of
formula units (fu). Finally, there are three ions (1 Mg2+ and two Cl-) per formula unit
of magnesium chloride.
? ions =
[0.1mg MgCl2 x (1 g MgCl2 / 1000 mg MgCl2) x (1 mol MgCl2 / 95 g MgCl2) x (6.0 x
10²³ fu MgCl2 / 1 mol MgCl2) x (3 ions / 1 fu MgCl2)]
= 2 x 1018 ions
Problem: The volatile liquid ethyl mercaptan, C2H6S, is one of the most odoriferous
substances known. It is sometimes added to natural gas to make gas leaks
detectable. How many C2H6S molecules are contained in a 1.0 µL sample? (d=0.84
? molecules C2H6S =