Stoichiometry :  Introduction
Jeremias Benjamin Richter
  • Defined Stoichiometry - The"art of chemical measurements, which has to deal with the laws according to which substance unite to form chemical compounds."  In other words, stoichiometry is the relationship between the weights of the reactants and the products of a chemical reaction. 
     [Image]  Atomic number: protons (and electrons if neutral) 

 Mass number: protons + neutrons (neutrons = mass number - atomic number)

The mass of an atom is measured in the atomic mass units (amu).

Mass of proton = 1.007276 amu1 amu

Mass of neutron = 1.008665 amu1 amu

Atomic weight - The weighted average of all isotopes of an atom based on each isotope's abundance.

Isotopic Mass
Percentage Composition
Average Atomic Weight
75.53 %
35.45 amu
24.47 %

The atomic weights are the mass values found on the periodic table.

Molecular weight - The sum of the atomic weights of the atoms in a molecule of a compound.

e.g.  The molecular weight of CH4 is the sum of the atomic weight of one carbon atom and four hydrogen atoms:

With the correct number of significant figures, the molecular weight of CH4 is 16.043 amu.  (See significant figures notes.)

Mole - A mole is, by definition, the amount of any substance that contains as many particles as there are atoms in exactly 12 grams of carbon-12.

1 carbon-12 atom = 12 amu

1 mol of carbon-12 atoms = 12 grams


Molar mass - The mass of a mole of a substance.  The molar mass of any element is equal to the atomic weight of the element.  The molar mass of any compound is equal to the molecular weight of the compound.  Since the molar mass equals the atomic weight, the mass values found on the periodic table are also the molar masses of the elements.
Avogadro's Number - The number of particles in a mole of any substance.
When solving stoichiometry problems (whether with one substance or more), the key is to convert the given information to moles because you can convert to any units from moles.