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

Molarity =

no. of liters of solution

MD0803

2-8

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