Untitled - Kelly Walsh High School

36 CHEMISTRY FOR THE UTTERLY CONFUSED
Don’t Forget!
You are starting with moles of iron and want grams of Fe 2O 3, so we’ll first convert
from moles of iron to moles of Fe 2O 3 using the ratio of moles of Fe 2O 3 to
moles of iron as defined by the balanced chemical equation:
(20.0 mol Fe)a 2 mol Fe2O3 159.689 g
ba b
4 mol Fe 1 mol Fe2O3 1.60 10 3 g Fe 2 O 3
The ratio of 2 mol Fe 2O 3 to 4 mol Fe is called a stoichiometric ratio, which
comes from the balanced chemical equation.
Suppose you also wanted to know how many grams of oxygen it would take to
react with the 19.9 mol of iron. All you would need to do would be to change the
stoichiometric ratio and the molar mass:
(19.9 mol Fe)a 3 mol O2 g
ba31.998 b 478 g O2 4 mol Fe 1 mol O2 Notice that this new stoichiometric ratio came from the balanced chemical
equation.
In working stoichiometry problems you will need the balanced chemical equation.
In addition, if the problem involves a quantity other than moles, you will
need to convert to moles.
3-4 Limiting Reagent and Percent Yield
In the examples above, we indicated that one reactant was present in excess.
The other reactant is consumed and there would be some of the reactant in
excess left over. The first reactant to react completely is the limiting reactant
(reagent). This reactant really determines the amount of product formed. There
are, in general, two ways to determine which reactant is the limiting reactant:
1. We assume each reactant, in turn, to be the limiting reactant and we calculate
the amount of product that forms. The reactant that yields the smallest
amount of product is the limiting reactant.