A mole is basically just a word for a number. In the same way we say one "hundred" menaing 100, or one "million" meaning one 1,000,000, we could also say one "mole" which also corresponds to a number. A mole is a very larger number indeed. In numbers, it would be written as 602213670000000000000000. Thus you can see why people refer to them as "moles" rather than writing out the entire number.
Theoretically, you can have a mole of anything you want. A mole of apples, a mole of oranges, or whatever. But because the number is so very large, it tends to only apply to thing that there are a lot of. For example, the number of grains of sand in a beach several miles long might be a mole or two. The number of leaves in a large forest might be a few moles. Usually, however, the mole is used in connection with atoms.
Different elements of the periodic table weigh different amounts. One atom of sodium, for example, is considerably lighter than one atom of chlorine. Suppose we wanted to combine sodium and chlorine to form table salt (sodium chloride). If we mix up 10 grams of sodium with 10 grams of chlorine, we will have large amounts of sodium leftover. This is because a chlorine atom is about one and a half times heavier than a sodium atom.Ten grams of soidum weighs the same as ten grams of chlorine, but there are more sodium atoms present than chlorine atoms, since chlorine atoms weigh more. (If I give you 10 pounds of bricks, you will not expect many bricks . . . but if I give you 10 pounds of feathers, that will be a lot of feathers).
In order to recat things in equal quantities, we need to have the same number of atoms, not the same weight. Ten atoms of sodium--no matter what they weigh--will react perfectly with ten atoms of chlorine, leaving us with no leftovers. But an atom is so incredibly tiny that even our most microscopic equipment could not hope to measure out ten atoms of anything. So instead, we work in moles of atoms. An atom is a very very tiny thing, but a mole of atoms (602213670000000000000000 atoms) is something large enough for us to see and work with easily. A mole of carbon atoms, for example, weighs about 12 grams. A mole of sodium atoms weighs about 23 grams. A mole of chlorine atoms weighs about 35.5 grams, and so on. By working in "moles of atoms" (abbreviated to just "moles") we are working on a scale that involves reasonable numbers.
How did I know a mole of carbon atoms was about 12 grams? This is not something you need to memorize. The weight for a mole of carbon atoms has been carefully calculated by scientists a long time ago, and worked out to be 12.011 grams. In fact, this has been done for all the elements currently known to humankind, and the results are printed on any Periodic Table as the "Atomic Weight" of the element. Working with a Periodic Table is essential for all mole calculations.
I. Going from mass to moles:
To go from mass to moles, simply divide the mass by the formula weight of the compound. The formula weight is determined by adding together the atomic weights (as found on the periodic table) of all elements present in the formula.
Example: 10.0 grams of NaCl
Answer: A periodic table will show you the atomic weight of Na is 22.9898. The atomic weight of chlorine is 35.4527 (again, found on the Periodic Table). So the formula weight of sodium chloride is 22.9898 + 35.4527 = 58.4425g/mol. To get moles of sodium chloride, divide mass by formula weight. 10.0g/58.4425g/mol = 0.1711 moles.
II. Going from moles to mass:
To go from moles to mass, multiply the moles by the formula weight of the compound.
Example: 0.50 moles of H2O
Answer: The atomic weight of hydrogen is 1.0079. Oxygen is 15.9994. So the formula weight of H2O is 15.9994 + (2 x 1.0079) = 18.0152g/mol. To get mass of water, multiply moles by formula weight. 0.50 moles x 18.0152g/mol = 9.0076g.
III. Going from moles to atoms:
Multiply moles by Avogadro's Number (6.022 x 10^23) to obtain atoms.
Example: How many atoms is 0.5 moles of lead?
Answer: 0.5 x (6.022 x 10^23) = 3.011 x 10^23 atoms.