Stoichiometry

 In the everday world of our experience, matter is measured in pieces that are scaled to our convenience- grams, handfuls, tons.  When investigating the chemical reactions responsible for air pollution however, we need to know the number of atoms or molecules that are present because it is these miniscule individual entities that are actually participating in a chemical reactions.  How can an environmental chemist predict the amount of sulfur released as a result of the combustion of a ton of coal?  The answer is by using chemical stoichiometry.  The term stoichiometry comes from the Greek stoicheion, "element or part," and metron, " to measure". Stoichiometry is the chemists tool for relating the mass of a substance to the number of atoms, molecules, or formula units present.  Using this tool,  you can move from a world scaled to humans down to the world scaled to the size of individual atoms and molecules.
 

The secret weapon of stoichiometry is the mole. The mole maintains the same mass relationship between macroscopic samples as exists between individual atoms or molecules. It relates the number of chemical units present to the mass of the units present.  The molar massof any substance is just the mass of 1 mole of that substance.
 

 

The Definition of Molar Mass for an Element, Molecule, or Formula Unit

Element	Molecule	Ionic Compound
Themolar mass of an elementis the mass of 1 mole of atoms of the element.	The molar mass of a molecular compound  is the sum of the masses of the number of moles of all its  elements.	The molar mass of an ionic compound is the mass of the compound per mole of its formula units.
For example:     the molar mass of Na is 22.99 g per mole of Na.	For example: the molar mass of C6H1002 is : 6 mol of C x (12.01 g C /mol) +10 mol H x (1.004 g H/mol) +2 mol O x (16.00 g O/mol) = 114.10 g/mol	For example: the molar mass of CaCl2 is:   1 mol of Ca x (40.08 g of Ca/ mol) + 2 mol Cl x (35.45 g Cl / mol) = 110.90 g/mol

Converting mass to moles is simplified using this general equation:  Mass (g) = # of moles  x  no. of grams/1 mole of substance.

This equation can be rearranged to solve for # of moles of a substance: # of moles = mass (g) x 1 mole of substance/grams in 1 mol of substance