Chapter 7
Chemical Quantities
Fall 2008
The Mole: A Measurement of MatterWhat Is a Mole?
 How do you measure matter?
 You count things
 You weigh things or determine mass
 You get the volume
 Can you count the grains of sand?
Would you want to?
 How do we determine the amount of
something big or small without having
to count it?
The Mole: A Measurement of MatterWhat Is a Mole?
 We use problem solving steps to figure
out the amount of very small or very big
forms of matter. That method is
called……
Dimensional Analysis
 Here is an example:
 How many eggs can Bug Bunny buy for
cupcakes for Elmer Fudd’s birthday if
he has $10 to buy eggs with at $0.80
cent for a dozen?
 First start with what you know
 12 eggs = 1 dozen, 6 eggs = ½ dozen
 Eggs cost ¢80 for 1 dozen
 Bugs has $10
Dimensional Analysis
 Next we set up Conversion factors
 12 eggs/ 1 doz. Or 1 doz./12 eggs
 $0.80/ 1doz. Or 1 doz./$0.80
 Now we can set up the problem
 $10
1 doz. 12 eggs = 150 eggs
$ 0.80
1 doz.
What is a Mole?
 But now back to the mole…
 In chemistry, it is not the furry little
creature…
 It is the measurement of the amount of
particles in a substance
 Particles can be atoms, ions, formula units or
molecules
What is a Mole?
 So, how much is a mole…..
6.0223 x 1023 particles
 Particles are extremely small and
that number is extremely big.
 This is due to Amadeo Avagadro’s
research leading to the discovery,
not the discovery of the number
itself.
What is a Mole?
 It has also been discovered that
1 mole of all gases at a certain
set of conditions called STP
(standard temperature and
pressure) have the same
volume—22.4L!
 So, 1 mole = 22.4L (gas @ STP)
Mass and Moles
 How do we usually measure out
chemicals in a lab?
 NOT by counting the number of particles
 We usually use mass to measure out
chemicals. The problem with mass is that
not all atoms or compounds have the same
mass.
 Avogadro’s number was established by using
Carbon-12 as a standard. The number of
particles in 12 grams of carbon 12 was set
as 1 mole.
Mass and Moles
 Conveniently, this is similar to how the
atomic mass unit was established, so the
atomic mass on the periodic table is also
an element’s molar mass (the mass of one
mole of an element, measured in g/mol).
 Since compounds are just atoms bonded
together, you can find the molar mass of a
compound by adding together the molar
mass of all the elements in the compound.
Mass and Moles
 We have now discussed moles in three
ways—as particles (1 mol = 6.02x1023
particles), as a mass ( molar mass = 1
mol) and as a volume (22.4 L = 1 mol)
 To get from particles to grams, grams
to liters or particles to liters of a
substance, two steps are required, with
moles always in between!
The Mole Map
Gas
Volume
@ STP

x
#
Particles
x

Mass

Molar
Mole
Mass
x
Mole Road Map
 Here is a graphic of the procedure steps:
 Pick the box of the data you are given in
the problem and follow the steps toward
the box containing what you are asked for
in the problem.
% Composition
 The percent composition of an
element is the relative amount of
that element in a compound.
 You find the percent composition of
an element by dividing the element’s
mass by the mass of the entire
compound.
mass of element
mass of compound
x 100
For example:
 What is the % composition of carbon
dioxide?
Mass of oxygen = 2(16.0)=32.0 g
Mass of carbon = 12.0 g
Mass of carbon dioxide = 44.0 g
12.0 g C
44.0 g CO2 x100 = 27.3% Carbon
32.0 g O
44.0 g CO2
x100 = 72.7% Oxygen
% Water in a Hydrate



A hydrate is an ionic compound with
water as part of its crystalline
structure.
To find the percent of water in a
hydrate, use the same formula,
except:
% Water = Mass water
x 100
Mass compound
Empirical Formula
 The empirical formula gives you the lowest,
whole number ratio of elements in the
compound.
 The empirical formula may or may not be
the same as the molecular formula.
 For carbon dioxide, the molecular formula
is CO2, and the empirical formula is CO2.
One carbon and two oxygens are the
lowest ratio of atoms.
 The molecular formula for dinitrogen
tetrahydride is N2H4, but the empirical
formula is NH2.
What is the empirical formula for:
C6H12O6
N2H2
CH4
To Find the Empirical Formula from %
Composition:
 If given the percentages, assume there are
100.0 grams of the compound.
 Convert the grams of each element to moles.
 Divide by the smaller amount of moles, then
manipulate the ratio so that all numbers are
whole.
 What is the empirical formula of a compound
that is 27.3% carbon and 72.7% oxygen?
 What is the empirical formula of a compound
that is 25.9% nitrogen and 74.1% oxygen?
To Find the Empirical Formula from %
Composition:
 If given the percentages, assume
100% = 100.0 grams .
 Convert the grams of each
element to moles.
 Divide by the smaller amount of
moles, then manipulate the ratio
so that all numbers are whole.
From there, find it’s molecular formula:
 If given the molar mass (how many g/mol
of the compound) then you can calculate
the molecular formula from the
empirical formula.
 Take the compound’s empirical formula
mass and compare to the molecular mass.
The molecular mass will be a multiple of
the empirical formula’s mass.