Quantitative Analysis
 Deals
with mass relationships of
elements in compounds
• Formula (molar) mass
• Converting grams to moles to atoms/molecules
Find the molar mass of:
Al2O3 =
Al =
O2 =
 Involves
the mass relationships between
reactants and products in a chemical
reaction
 Based
on balanced chemical equations
and the law of conservation of mass
 Mole
ratio = a conversion factor that
relates the amounts in moles of any two
substances involved in a chemical rxn
Amt (moles) of given → amt (moles) of unknown
Ex.
For every 2 moles of Al2O3 ,
you produce 4 moles of Al and
3 moles of O2
2Al2O3 → 4Al + 3O2
The coefficients represent the mole
ratios of all elements and compounds
in a balanced equation.
EXAMPLE: How many moles of oxygen will be produced if
you decompose 5.8 moles of aluminum oxide?
Amt of given→amt of unknown→mass of unknown
2Al2O3 → 4Al + 3O2
For every 2 moles of Al2O3 ,
you produce 4 moles of Al and
3 moles of O2
EXAMPLE: What was the mass of
aluminum oxide decomposed if
5 moles of oxygen is produced?
Mass of
given
Moles of
given
2Al2O3 → 4Al + 3O2
If 150 grams of aluminum oxide is
decomposed, how many moles of
aluminum are produced?
Moles of
unknown
Mass of
given
Moles of
given
Moles of
unknown
Mass of
unknown
2Al2O3 → 4Al + 3O2
If 300 grams of aluminum oxide is
decomposed, what is the mass of
aluminum produced?
You have just
calculated the
theoretical yield
of this reaction.
 Theoretical
• Amount calculated
based on balanced
equation and starting
amount of reactant
 Actual
• Amount actually
produced in the lab
when the reaction is
observed.
actual
%yield =
´100
theoretical
 Exact
required amount of reactants based
on equation?
 Not
 We
realistic
will run out of supplies of one of the
reactants and have excess of the others
= the reactant that limits the amount of the
products that can be produced (because
we have a limited amt)
Excess reactant = the substance that is
NOT used up completely
Given: equation and masses of reactants
Unknown: limiting reactant
1.
2.
3.
Write and balance chemical equation
Convert masses (g) of reactants into moles of
reactants using their molar masses
Multiply mole amount of each reactant by
mole ratio to determine mole amount of
products
Whichever reactant produces the least
of the product is the limiting reactant.
N2H4 (l) + H2O2 (l) → N2 (g) + H2O (g)
Which is the limiting reactant when 0.750 mol of
N2H4 is mixed with 0.500 moles of H2O2?
Fe (s) + H2O (g) → Fe3O4 (s) + H2 (g)
When 36.0 g of water is mixed with 67.0 g
of iron, which is the limiting reactant?
After limiting reactant is identified, you can
use it to determine theoretical yield

Use mole amount of limiting reactant and
mole ratio of equation to determine how
much product is produced (just like you
did last week)
g given
mol given
mol unknown
g unknown
Zn (s) + S8 (s) → ZnS (s)
2.00 mol Zn reacts with 1.00
mol of S8
What is the limiting
reactant?
 How many moles of the
product are produced
(what is the theoretical
yield)?

2.40 mol of C react with
3.10 mol of steam
 What
is the limiting
reactant?
 Calculate theoretical
yield of each product
When you can calculate the moles used or
produced, you can always convert it into
grams USING MOLAR MASS!!!
When you are given the mass of
something, you MUST FIRST convert it
into moles (USING MOLAR MASS!!)
before you can begin a problem
Grams → Moles → Moles → Grams