Download Molecular Stoichiometry

Mole Relationships
Atomic mass is the mass of an atom in
atomic mass units (amu)
By definition:
1 atom 12C “weighs” 12 amu
The mole (mol) is the amount of a substance that
contains as many elementary entities as there
are atoms in exactly 12.00 grams of 12C
eggs
Molar mass is the mass of 1 mole of shoes in grams
marbles
atoms
1 mole 12C atoms = 6.022 x 1023 atoms = 12.00 g
1 12C atom = 12.00 amu
1 mole 12C atoms =
1 mole lithium atoms =
g 12C
g of Li
For any element
atomic mass (amu) = molar mass (grams)
One Mole of:
S
C
Hg
Cu
Fe
Do You Understand Molar Mass?
How many atoms are in 0.551 g of potassium (K) ?
Molecular mass (or molecular weight) is the sum of
the atomic masses (in amu) in a molecule.
1S
SO2
2O
SO2
32.07 amu
+ 2 x 16.00 amu
64.07 amu
For any molecule
molecular mass (amu) = molar mass (grams)
Do You Understand Molecular Mass?
How many H atoms are in 72.5 g of C3H8O ?
Percent composition of an element in a compound =
n x molar mass of element
x 100%
molar mass of compound
n is the number of moles of the element in 1 mole
of the compound
C2H6O
Do You Understand Percent composition ?
What is the percent composition of each
element in C3H8O ?
Types of Formulas
• Empirical Formula
The formula of a compound that
expresses the smallest whole number
ratio of the atoms present.
Ionic formula are always empirical formula
• Molecular Formula
The formula that states the actual
number of each kind of atom found in one
molecule of the compound.
To obtain an Empirical Formula
1. Determine the mass in grams of each
element present, if necessary.
2. Calculate the number of moles of each
element.
3. Divide each by the smallest number of
moles to obtain the simplest whole
number ratio.
4. If whole numbers are not obtained* in
step 3), multiply through by the smallest
number that will give all whole numbers
* Be
careful! Do not round off numbers prematurely
To obtain an Empirical Formula
A sample of a brown gas, a major air pollutant,
is found to contain 2.34 g N and 5.34g O.
Determine an empirical formula for this
substance.
To obtain a Molecular Formula
Calculation of the Molecular Formula
A compound has an empirical formula of
NO2. The colourless liquid, used in rocket
engines has a molar mass of 92.0 g/mole.
What is the molecular formula of this
substance?
Empirical Formula from % Composition
A substance has the following composition by
mass: 60.80 % Na ; 28.60 % B ; 10.60 % H
What is the empirical formula of the substance?
Molecular Formula from % Composition
A substance has the following composition by
mass: 60.80 % Na ; 28.60 % B ; 10.60 % H
What is the molecular formula of the substance if
the molecular mass is 151.2 g/mol?
Combustion Analysis
Many times it is difficult to determine the mass of
individual elements. We can use combustion
analysis of organic elements to determine the
empirical formula for these compounds
1. Find total mass combusted
2. Find mass water using water absorption
3. Find mass carbon dioxide through CO2
absorption
4. Find remaining mass of other elements if needed.
Combust 11.5 g ethanol
Collect 22.0 g CO2 and 13.5 g H2O
Combust 11.5 g ethanol
Collect 22.0 g CO2 and 13.5 g H2O
If molar mass was found to be 46 g/mole what is the molecular formula?
Do You Understand Combustion analysis?
What is the empirical formula if 31.81 g CO2 and
11.39 g H2O were collected from a 15.0 g sample
of an organic molecule containing carbon
hydrogen and nitrogen?