Stoichiometry: Chemical Formulas and Equations

The Mole


The Mole

Even tiny samples of chemicals contain huge numbers of atoms, ions or molecules. For convenience sake, some kind of reference for a collection of a large number of these objects would be very useful (e.g. a "dozen" is a reference to a collection of 12 objects). In chemistry we use a unit called a mole (abbreviated mol).

A mole is defined as the amount of matter that contains as many objects as the number of atoms in exactly 12 grams of 12C.

Various experiments have determined that this number is...

6.0221367 x 1023

This is usually abbreviated to simply 6.02 x 1023, and is known as Avogadro's number.

One mole of atoms, volkswagens, people, etc. contains 6.02 x 1023 of these objects. Just how big is this number? One mole of marbles spread over the earth would result in a layer three miles thick.

Molar Mass

A single 12C atom has a mass of 12 amu. A single 24Mg atom has a mass of 24 amu, or twice the mass of a 12C atom. Thus, one mole of 24Mg atoms should have twice the mass as one mole of 12C atoms. Since one mole of 12C atoms weighs 12 grams (by definition), one mole of 24Mg atoms must weigh 24 grams.

Note that the mass of one atom in atomic mass units (amu) is numerically equal to the mass of one mole of the same atoms in grams (g).

The mass in grams of 1 mole (mol) of a substance is called its molar mass.

The molar mass (in grams) of any substance is always numerically equal to its formula weight (in amu).

One H2O molecule weighs 18.0 amu; 1 mol of H2O weighs 18.0 grams

One NaCl ion pair weighs 58.5 amu; 1 mol of NaCl weighs 58.5 grams

Interconverting masses, moles, and numbers of particles

Keeping track of units in calculations is necessary when interconverting masses and moles. This is formally known as dimensional analysis.


"Igor! bring me 1.5 moles of calcium chloride"

Chemical formula of calcium chloride = CaCl2

Molecular mass of Ca = 40.078 amu

Molecular mass of Cl = 35.453 amu

Therefore, the formula weight of CaCl2 = (40.078) + 2(35.453) = 110.984 amu (remember, this compound is ionic, so there is no "molecular" weight).

Therefore, one mole of CaCl2 would have a mass of 110.984 grams.

so, 1.5 moles of CaCl2 would be:

(1.5 mole)(110.984 grams/mole) = 166.476 grams


"Igor! I have 2.8 grams of gold, how many atoms do I have?"

Molecular formula of gold is: Au

Molecular weight of Au = 196.9665 amu

Therefore, 1 mole of gold weighs 196.9665 grams. So, in 2.8 grams of gold we would have:

(2.8 gram)(1 mole/196.9665 gram) = 0.0142 mole

From Avogadro's number, we know that there are approximately 6.02 x 1023 atoms/mole. Therefore, in 0.0142 moles we would have:

(0.0142 mole)(6.02 x 1023 atoms/mole) = 8.56 x 1021 atoms


1996 Michael Blaber