Download Introduction to Stoichiometry

Stoichiometry
The mathematics of Chemistry
What is Stoichiometry?
The proportional relationship between two
or more substances during a chemical
reaction.
 In other words, using dimensional analysis
to convert one substance to another
 There are many different types, but they
are all similar.
 So, let’s start small. How small?

We know an atom is
extremely small, but how
small is an atom?
In other words, how small is the
smallest particle of an element that
retains the properties of the element?
World Population: about 6000000000
Number of atoms in a penny:
about 29700000000000000000000 or 2.97 x 1022
How do we get that
number?
By using Avogadro's Number
6.02 x 1023
and using dimensional analysis
again
What is Avogadro’s Number?
It is 6.02 x 1023 atoms per mole
 What is a mole?
 The number of atoms in exactly 12 grams
of carbon-12 and it is the SI unit for the
amount of a substance
 Example problems

– 6.33 x 1024 atoms of C are how many
moles?
– How many atoms of Zn are in 3.5 moles?
The mole is also related to
molar mass
Molar Mass – the mass in grams of 1 mol
of an element, ion, compound, etc.
Represented by g/mol (what we did last
class)
 Example problems

– 92.6 g of CO2 equals how many moles?
– 1.25 moles of S8 equals how many
grams?
Atomic Mass Units verses Molar
Mass
The mass in grams of 1 mol of an element
is numerically equal to the element’s
atomic mass from the periodic table in
atomic mass units.
 In other words, 1 g/mol = 1 amu
 They are interchangeable

Putting it all together. Let’s look
at our penny again
How did they get that number for the
number of Cu atoms in a penny?
 A penny has an average mass of 3.13 g.
 Copper weighs 63.546 amu (g/mol), which
we found on the periodic table


Here’s the game plan. We want to go
from grams to atoms, but we have to
always go through the mole. So, we will
go from g  moles  atoms
Example Problems
3.13 g Cu x
0.049255 mol Cu x
1 mol Cu
63.546 g Cu
6.02 x 1023
1 mol Cu
= 0.049255 mol Cu
= 2.97 x 1022 Cu atoms
So, this is doing two types
of Stoichiometry Problems.
Atoms  moles
Grams  moles
Summary Slide #1
How is a mole related to atoms?
What is the magic number?
 Avogadro’s number (NA)– 6.02 x 1023
atoms (molecules, ions, whatever)/mole
 When going from moles to atoms
(multiply):
6.02 x 1023 atoms
1 mol
 When going from atoms to moles (divide):
1 mol
6.02 x 1023 atoms

Summary Slide #2
How is the mole related to mass?
Molar Mass
 For an element, it is the mass from the
periodic table
 For compounds, you must calculate the
molecular weight of the compound.
 Sometimes, before one can work with a
mass, you have to use the density of the
substance. (D=m/V or m=DV)

Summary Slide #3
How is a mole related to mass?

When going from moles to grams
(multiply):
molar mass
1 mol

When going from grams to moles (divide):
1 mol
molar mass
Example Problems
1)
2)
3)
4)
5)
6)
7)
How many moles are in 10.23 g of PO4-3?
What is the mass of 6 moles of CH2O?
How many ions are in 0.25 moles of Cu+2?
How many moles are 8.25 x 1028 molecules of
Na2CO3?
How many formula units are in 6.34 g of NaCl?
How many grams are in 3.4 x 1024 molecules of
CH4?
How many moles are in 10 mL of water (density of
water = 1 g/mL)?
Homework

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