Stoichiometry:
Measurement of
Elements
Formula Weights
Sum of the atomic weights (amu)
of each atom in a chemical
formula.
Called “molecular weight” or
“molecular mass” for a molecule.
Use “formula weight” for ionic
cmpds.
The Mole
Chemists have derived a special
counting unit for describing very large
numbers of atoms or molecules.
Mole (mol) = the amount of matter that
contains as many objects as the number
of atoms in exactly 12g of isotopically
pure carbon-12.
Mole, cont.
 Avogadro’s number: 6.022 x 10^23
 A mole of ANYTHING contains 6.022 x 10^23
objects!
 I mole of marbles would cover the entire surface
of the earth in a layer ________ thick.
 Pennies placed side to side in a straight line
would encircle the earth ________ times.
Representative Particles
Species present in a substance.
Substance
Type of
Species
Representative Representative
Particles
Particles in
1.00 mole
Lithium
element
atom
6.02 x 10^23
Nitrogen
gas
Calcium
fluoride
Calcium
ion
molecular
compound
ionic
compound
ion
molecule
6.02 x 10^23
formula unit
(f.u.)
ion
6.02 x 10^23
6.02 X 10^23
Moles, cont., cont.
Remember:
1 mol of carbon = 6.022 X 10^23 atoms.
1 mol water = ____ molecules.
1 mol CN- ions = _____ ions.
1 mol NaCl = ____ formula units (f.u.’s).
1 mol elephants = _____ elephants.
Mole-Particle/Particle-Mole
Problems
Use dimensional analysis!
How many moles of magnesium are
1.25 x 10^24 atoms of magnesium?
How many moles are 2.80 x 10^24 atoms Si?
How many molecules are in 3.60 mol water?
How many hydrogen atoms are in 3.60 mol
water????
Molar Mass
A mole is always the same number but
moles of different substances have
different masses.
1 mol water = 18.02 g; 1 mol lead =
207.2 g
Molar mass: the mass in grams of 1
mole of a substance. Unit is g/mol.
Calculating Molar Mass
Calculate molar mass of CH4.
C: 1 atom x 12.01 g/mol = 12.01 g/mol
H: 4 atoms X 1.01 g/mol = 4.04 g/mol
Add together:
12.01g/mol + 4.04 g/mol = 16.05 g/mol
Watch your sig figs!!!
Practice Calculating Molar Mass
Calculate the molar mass of NaCl.
Calculate the molar mass of calcium
hydroxide.
Calculate the molar mass of Sr3(PO4)2.
Do you like this more than
nomenclature?
Mass-Mole/ Mole-Mass Problems
Use dimensional analysis!
Find the mass of 4.52 x 10^-3 mol water.
Find the mass of 0.342 mol nitrogen gas.
How many moles are in 72.96 g LiCl?
How many moles are in 12 28 g KOH?
Are we having fun yet?
Mole-Volume Relationship
Avogadro’s Hypothesis: equal volumes
of gases at the same temp and pressure
contain an equal number of particles.
Note: the size or mass of gas particles
does NOT matter because particles are
so far apart from each other.
Molar Volume of a Gas
STP = standard temp and pressure
Standard temp for gases = 0°C
Standard pressure = 1 atm
“Molar volume of a gas” means that at
STP…
I mole of ANY gas = 22.4L of gas
Mole-Volume Problems
We use molar volume to do mole-volume
problems! Yeah!
Calculate the volume of 3.20 x 10^-3 mol
carbon dioxide at STP.
0.0717 L carbon dioxide
Calculate the volume of 3.70 mol nitrogen
gas at STP.
82.9 L nitrogen gas
More Molar Volume Calculations!
You have 0.200 L hydrogen gas at STP.
How many moles?
8.93 x 10^-3 mol
Dan has 0.375 mol of oxygen gas at
STP. What is its volume?
8.40 L oxygen
Density & Molar Volume at STP
The density of gases is in g/L,
NOT g/mL. WHY?
You can use density along with
molar volume to get molar
mass! Hurrah!
Density & Molar Volume Problems
 A gas has a density of 3.58 g/L at STP. What
is the molar mass of the gas?
 80.2 g/mol
 A gas has a density of 1.964 g/L at STP. What
is its molar mass?
 43.99 g/mol
 What is the density of krypton gas at STP?
 3.74 g/L
Percentage Composition
from Chemical Formula
Percentage by mass of each
element in a substance.
% element = (# of atoms x molar
mass of element/ molar mass of
compound) x 100
Total % of each element = 100%
Calculating Percentage
Composition
Calculate the % composition of C6H12O6.
% C = (6 x 12.01 g/mol/180.2 g/mol) x 100
= 40.00%
% H = (12 X 1.01 g/mol/180.2 g/mol) x 100
= 6.73%
% O = 100.00% - 40.00% - 6.73%
= 53.27%
Percent Composition Practice
Calculate the percent composition of
C3H8.
C: 81.8 %; H: 18.2 %
Calculate the percent on nitrogen in
NH3.
82.4% nitrogen
Percent Composition from
Mass Data
Percent Composition = (mass of
element in cmpd/mass of cmpd) x
100
Total %’s of all elements in cmpd =
100%
Percent Composition by Mass
Practice
When a 13.60-g sample of a cmpd
containing only Mg and O is
decomposed, 5.40 g oxygen is obtained.
What is the percent comp of the cmpd?
O: (5.40 g O/ 13.60 g) x 100 = 39.7%
Mg: [(13.60 g- 5.40 g)/ 13.60 g] x 100 =
60.3%
More Percent Composition
Practice
Calculate the number of grams of
sulfur in 3.54 g H2S.
First, find the % of sulfur in H2S.
94.1% S
Then, multiply .941 X 3.54 g H2S.
3.33 g
Empirical & Molecular
Formulas
Empirical formula: gives the
lowest whole number ratio of
atoms in a compound. PbO, not
Pb2O2
Molecular formula: whole number
multiples of the empirical formula.
Determining Empirical
Formulas
Sing the Empirical Formula Song
(sung to tune of “Twinkle, Twinkle
Little Star”):
“Percent to mass,
mass to mole,
divide by small,
multiply ‘til whole.”
Empirical Formula Practice
A compound is analyzed and found
to contain 25.9% nitrogen and
74.1% oxygen. What is the
empirical formula of the compound?
N2O5
More Empirical Formula
Calculations!
Find the empirical formula for the
following cmpd: Hg: 67.6%; S:
10.8%; O: 21.6%
HgSO4
Find the empirical formula for the
following cmpd: H: 5.9%; O: 94.1%
HO
Molecular Formulas
Once you have determined the empirical
formula (e.f.), you can determine the
molecular formula (m.f.) if you know the
compound’s molar mass.
Divide the molar mass of the cmpd by
the molar mass of the e.f. and get a
whole number. Multiply this number by
each subscript in the e.f. to get the m.f..
Finding a Molecular Formula
You have the empirical formula HO. Find
the molecular formula if its molar mass is
34.0 g/mol.
34.0 g/mol/ 17.0 g/mol = 2
The empirical formula HO becomes the
molecular formula H2O2 !
Finding Molecular Formulas :-)
Find the molecular formula of a cmpd
with a molar mass of 60.0 g/mol and an
empirical formula of CH4N.
C2H8N2
Find the m.f. of a cmpd with a m.m of
62.0 g/mol and an e.f. of CH3O.
C2H6O2