The Mole

Now we will return to the comparison of atomic masses.

Having developed a relative scale for comparing masses of atoms, the next logical step is to produce a similar scale that we may actually use in the laboratory. (Don’t forget that individual atoms are far too small to weigh!)

It would be logical and convenient if we could develop a second scale which had the same numbers as the relative atomic mass scale, but which had useable units. This is quite possible if we consider packages of atoms rather than individual atoms. A simple analogy is that when we buy eggs in the supermarket, we buy them by the dozen. Chemists need their own ‘dozen’ to describe ‘packages’ of atoms.

The chemist’s ‘dozen’ is the mole and is the quantity of atoms required to express all of the masses on the relative atomic mass scale in grams.

For example, 1 mole of carbon atoms would have a mass of 12.0000g (by definition).

1 mole of zinc atoms would have a mass of 65.4g and so on.

Experimentation has determined that a mole is a very large package indeed,

6.02 x 10²³ units.

ie, 1 mole of carbon atoms contains 6.02 x 10²³ atoms

1 mole of zinc atoms contains 6.02 x 10²³ atoms.

1 mole of methane molecules contains 6.02 x 10²³ molecules.

This large number is one of the most commonly used numbers in Chemistry and is called Avogadro’s Number (after the Italian scientist Amadeo Avogadro).

The mole is a number that answers the question, "How many?", in the same way a metre answers the question, "How far?", or a kilogram answers the question, "How much mass?"

A mole is the number of atoms in the number of grams of the relative atomic mass of an element. Thus one mole of Chlorine-35 atoms has a mass of 35 grams, one mole of Chlorine-37 atoms has a mass of 37 grams, and one mole of Chlorine atoms selected at random has a mass of 35.453 grams. This is the mass number printed on your periodic table. Notice the following:

It is a weighted average.If 75% of all chlorine atoms are Cl-35 and 25% are Cl-37, then the weighted average is (0.75 × 35) + (0.25 × 37) = 35.5.
You can see that either these percentages are rounded, or small amounts of other isotopes have been ignored, or something even more complex. [LINK about p. 203 in text.]

For example, sodium chloride has the formula NaCl. When 6.02 x 10²³ sodium ions and 6.02 x 10²³ chloride ions combine together we have 1 mole of sodium chloride. This means that:

One mole of NaCl contains one mole of sodium ions and one mole of chloride ions.

One mole of Na is the same amount as 22.9898 grams.

One mole of Cl is the same amount as 35.453 grams.

One mole of NaCl is the same amount as 22.9898 + 35.453 grams, or 58.4418 grams. It follows that:

1/2 mole of NaCl is 29.2209 grams.
2 mole of NaCl is 104.8836 grams.
10 mole of NaCl is 584.418 grams.
And so forth.

Note: The mass of an electron is negligible compared to the rest of the atom.

(To help you understand this idea think of a cup of coffee: 1 cup and 1 saucer with 1 spoon of coffee and 1 measure of hot water makes one cup of hot coffee.)

Compare this to another ionic compound, K₂O. This compound contains two moles of potassium ions, K⁺, and one mole of oxide ions, O^2-, in one mole of potassium oxide.

How many moles of chromium ions and oxide ions are present in one mole of Chromium (III) oxide, Cr₂O₃?

Avogadro's "Number" is now Avogadro's CONSTANT. With improvements in the ability to measure mass, the old standard kilogram which is a lump of metal in Paris, is no longer sufficient. The kilogram is now being measured in terms of a DEFINED Avogadro's number and very sophisticated techniques of counting atoms. This is like the metre being measured from the speed of light.

Next Page: Relative Molecular Mass

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