STOICHIOMETRY

 

Stoichiometry is the part of chemistry that studies amounts of substances that are involved in reactions. You might be looking at the amounts of substances before the reaction. You might be looking at the amount of material that is produced by the reaction. Stoichiometry is all about amounts.

What do you measure? It could be anything. When you're doing problems in stoichiometry, you might look at...

• Mass of Reactants (chemicals before the reaction)
• Mass of Products (chemicals after the reaction)
• Chemical Equations
• Molecular Weights of Reactants and Products
• Formulas of Various Compounds

Now an example. Let's start with something simple like Sodium chloride (NaCl). You start with two ions and wind up with an ionic compound (NaCl). When you look at the equation, you see it takes one atom of sodium (Na) to combine with one atom of chlorine (Cl) to make the salt. When you use stoichiometry, you can determine amounts of substances needed to fulfill the requirements of the reaction. Stoichiometry will tell you that if you have ten million atoms of sodium (Na) and only one atom of chlorine (Cl) you can only make one molecule of sodium chloride (NaCl). Nothing you can do will change that.

Example:

A piece of metallic iron was dissolved in concentrated HCl. The reaction formed H₂ and FeCl₂.

Balanced equation is -->    Fe +  2HCl =  FeCl₂  +  H₂

_{How many grams of Hydrogen gas will be produced when 300 g of Iron metal is reacted with sufficient HCl?}

_{Solution -->    300 g  x  1 mole  x  1  mole of H2  x  2  g   =  10.8 g of H2} 

                                   _{55.8 g      1 mole of Fe      1 mole}