Download 1 mol H 2

Section 6.5—Stoichiometry
Can we determine, in the lab, the concentration of electrolytes?
Objectives
•Perform mole to mole stoichiometric
calculations using mole ratios from balanced
chemical equations
•Explain and use titrations, indicators, and
stoichiometry to determine the concentration of
an unknown acid
What is stoichiometry?
A chemist knows that a chemical reaction
stops when one of the reactants is used
up.
Therefore, the amounts of other
reactants used or products formed
depends on the amount of reactant that
is used up.
Stoichiometric calculations allow us to
use this information to determine the
amounts of other reactants used or
products formed in a reaction.
Stoichiometry is the study of
quantitative relationships between
amounts of reactants used and the
products formed by a chemical reaction.
It is based on the Law of Conservation
of Mass: the amount of matter present
at the end of a reaction is the same as
was present at the beginning.
The total mass of the reactants equals
the mass of the products.
Interpretation of Chemical Equations
•The coefficients in a chemical equation
can be interpreted several ways.
•We have learned to interpret them in
terms of representative particles.
Coefficients can also represent “number of
moles”!
We can, therefore, interpret coefficients as
numbers of moles!
What do those coefficients really mean?
For every 2 moles
of H2…
and 2 moles of H2O are
produced
2
2
2 H2 + O2  2 H2O
No coefficient = 1
1 mole of O2 is
need to react…
Mole Ratios
In the following reaction, how many
moles of Al will react with 3 moles of
Br2? How many moles of AlBr3 are
formed?
2Al(s) + 3Br2(l) → 2AlBr3(s)
The relationships above can be
expressed as MOLE RATIOS.
A mole ratio is a ratio between the
numbers of moles of any two
substances in a balanced chemical
equation.
Mole Ratios
2Al(s) + 3Br2(l) → 2AlBr3(s)
For example, the mole ratios between
Al and AlBr3 are:
2 mol Al and 2 mol AlBr3
2 mol AlBr3
2 mol Al
Mole Ratios
Mole ratios are used as conversion
factors in stoichiometric calculations
With the chemical equation and the
mole ratios, you can calculate the
amount used of any reactant in the
equation and the maximum amount of
product you can obtain.
Mole to Mole Conversions
Problem: Potassium reacts with water to
produce potassium hydroxide and
hydrogen gas. Determine the number
of moles of hydrogen gas produced
when 0.04 mol of K is used?
2K(s) + 2H2O(l)  2KOH(aq) + H2(g)
Mole to Mole Conversions
 Step 1: Write a balanced equation if you are
not given one.
2K(s) + 2H2O(l)
2KOH(aq) + H2(g)
 Step 2: Identify the known/given and the
unknown in the problem.
0.04 mol K is given; mol H2 is unknown.
 Step 3: Use the balanced equation to
determine the mole ratio in which mol
unknown is the numerator and mol known is
the denominator.
mol unknown = 1 mol H2
mol known
2 mol K
Mole to Mole Conversions
Step 4: Multiply the known by the mole
ratio. Note that known substance & its
units will cancel if it is correctly done.
0.04 mol K x 1 mol H2 = 0.02 mol H2
2 mol K
We have gone from moles of known
to moles of unknown!
Practice Problems
How many moles of carbon dioxide gas
are produced when 10.0 mol of propane
(C3H8) are burned in an excess of
oxygen?
C3H8 + 5 O2  3CO2 + 4H2O
If 4.2 mole of H2 reacts completely with
O2, how many moles of O2 are needed?
2 H2 + O2  2 H2O
But we can’t measure moles in lab!
We can’t go to the lab and count or measure
moles…so we need a way to convert from
moles to units we CAN measure in the lab,
such as grams and liters!
Remember molar mass?? It is the number
of grams in 1 mole of a substance. Using
molar mass, we can convert from moles to
a mass!
Stoichiometry with Moles & Mass
Example:
How many grams of AgCl will form if
0.45 mole AgNO3 is reacted as follows:
2 AgNO3 + CaCl2  2 AgCl + Ca(NO3)2
Stoichiometry with Moles & Mass
Example:
How many grams of AgCl will form if
0.45 mole AgNO3 is reacted as follows:
2 AgNO3 + CaCl2  2 AgCl + Ca(NO3)2
Step 1: Use stoichiometry to find mol AgCl from
0.45 mol AgNO3
0.45 mol AgNO3 x 2 mol AgCl
= 0.45 mol AgCl
2 mol AgNO3
Stoichiometry with Moles & Mass
Step 2: Convert the mol AgCl to grams using the
molar mass
1 Ag = 107.9
1 Cl = 35.5
143.4
0.45 mol AgCl x 143.4 g = 65 g
1 mol
Stoichiometry with Moles & Mass
Example:
How many grams of Ba(OH)2 are
formed from 5.2 mol of NaOH in the
following reaction:
2 NaOH + BaCl2  Ba(OH)2 + 2 NaCl
Stoichiometry with Moles & Mass
Example:
How many grams of Ba(OH)2 are
formed from 5.2 mol of NaOH in the
following reaction:
2 NaOH + BaCl2  Ba(OH)2 + 2 NaCl
Step 1: Go from 5.2 mol NaOH to moles Ba(OH)2
5.2 mol NaOH X 1 mol Ba(OH)2
2 mol NaOH
= 2.6 mol Ba(OH)2
Stoichiometry with Moles & Mass
Step 2: Convert 2.6 mol Ba(OH)2 to mass
Ba = 137.3
2 O = 32.0
2 H = 2.0
171.3
2.6 mol Ba(OH)2 X 171.3 g = 445 g
1 mol
Practice Problems
4Fe + 3O2  2Fe2O3
1.How many grams of iron must be used to
produce 3.0 mol of Fe2O3?
2. How many grams of oxygen are required to
react with 4.2 mol of Fe?
3.How many grams of Fe2O3 are formed when
0.85 mol of Fe are oxidized?
4. How many grams of oxygen are used to
produce 1.8 mol of Fe2O3?
What about gases?
One mole of any gas (at the standard
temperature of 0 oC and pressure of 1 atm)
will occupy a volume of 22.4 L.
So, for gases, we can determine the volume of
a gas from its number of moles. This is
sometimes more helpful to scientists than the
mass of the gas!!
Stoichiometry with Gases
Example:
If 1.5 mol of zinc reacted, what volume of
H2 gas will be produced at STP?
2 HCl (aq) + Zn (s)  ZnCl2 (aq) + H2 (g)
Stoichiometry with Gases
Example:
If 1.5 mol of zinc reacted, what volume of
H2 gas will be produced at STP?
2 HCl (aq) + Zn (s)  ZnCl2 (aq) + H2 (g)
Step 1: Go from 1.5 mol Zn to mol H2
1.5 mol Zn x 1 mol H2 = 1.5 mol H2
1 mol Zn
Stoichiometry with Gases
Step 2: Convert 1.5 mol H2 into Liters
1.5 mol H2 x 22.4 L = 33.6 L H2
1 mol
Practice
2H2 (g) + O2 (g)  2H2O (l)
1.How many liters of hydrogen are required to
produce 8.4 mol of water?
2.How many liters of oxygen are required to
produce 0.25 mol of water?
Keeping all these equalities straight!
TO GO BETWEEN
USE THE EQUALITY
Grams & moles
Molar Mass in grams = 1 mole
Moles & liters of a gas at
STP
1 mole = 22.4 L at STP
moles of 2 different
chemicals in a reaction
Coefficient ratio from balanced
equation