problem by learning how to count particles indirectly. They did this by measuring
samples of the chemicals in particular ratios by their weights. To understand the means
of doing this, we need to expand our concept of atomic weight to compounds in the form
of the formula (or molecular) weight.
a. Milligram Formula (Milligram Molecular) Weight. When atoms combine to
form compounds, the atomic nuclei are not affected. There is no net loss of weight.
Regardless of whether the particle formed is a molecule or an ion group, it will have a
formula and a formula weight. The formula weight of a compound is the sum of the
atomic weights of all the atoms that appear in its chemical formula. Consider, for
example, carbon dioxide:
Atoms: C + O + O = CO2 (molecule)
Atomic weights: 12 + 16 + 16 = 44 (formula weight)
While we have arrived at a formula weight which is in terms of atomic mass units, it is
much more useful to express it in terms of milligrams. This is known as the milligram
formula weight. For the example above, CO2, the milligram formula weight is 44 mg.
This is a quantity that we can measure and see, and thus can easily work with. It also
represents a reacting unit of the compound.
b. Molarity. A molar solution, or a one molar (1M) solution, consists of one-
gram molecular weight (GMW) of solute dissolved in enough water to make 1 liter of
finished solution. Molarity, then, is the number of GMWs dissolved in enough water to
make a finished solution of 1000 ml. Molar solutions may have as a solute a solid, a
liquid, or a gas. Later in this subcourse, we will use the concept of molarity to explain
the measurement of acidity, called the pH.
(1) Calculating the gram molecular weight. One-gram molecular weight of a
substance is its molecular weight expressed in grams. Thus, a GMW of NaOH would
be 40 grams, where the atomic weights are as follows: Na = 23, O = 16, and H = 1.
Thus, .5 GMW of NaOH would be 20 grams, and so forth. A mole is one-gram
molecular weight of a substance. Thus, a mole of NaOH is 40 grams of NaOH; a half-
mole (.5 mole) is 20 grams; two moles of NaOH are 80 grams, and so on.
(2) Calculating the molarity of a solution. To find the molarity of a solution,
we divide the number of gram molecular weights of solute by the number of liters of total
solution. The formula may be written:
no. of GMWs of solute
no. of liters of solution