PERCENT COMPOSITION
Text 4.5: Page 178-184
Agenda
1.
2.
3.
Homework Review
Percent Composition: Fertilizer Use
Teacher-Led Discussion
 Percent
Composition
 Calculated by Mass
 Calculated by Chemical Formula
4.
5.
Cookie Chemistry
Homework Assignment
Learning Goal

Students will be able to calculate the percent
composition from
 Masses
 Chemical
formulas
Fertilizer on Sale!
Percent Composition

Is the mass percent of a
element in a compound
 How
much one thing is made of
another

There are 2 ways we can
calculate percent composition:
 Experimentally
 Theoretically
Experimentally

EX: 2H + O  H2O

Determine the mass of the products &reactants
 MH=
2.5 g
 MO=
 MH20=
22.5g
Experimentally

Determine the Percent Composition
% Composition x = (Mx/ Mproduct) x 100
If MH= 20.0 g and MH20= 22.5g
Theoretically


EX: Na2CO3
Determine the mM of each element
 mM Na =
 mM C
=
 mM O
=
Theoretically


EX: Na2CO3
Determine the mM of the compound
mM Na = 22.99 g/mol
mM C = 12.01 g/mol
mM O = 16.00 g/mol
mM Na2CO3 = 105.99 g/mol
Theroetically

Determine the % composition
% Composition x = (mMx/ mMproduct) x 100
mM Na = 22.99 g/mol
mM C = 12.01 g/mol
mM O = 16.00 g/mol
mM Na2CO3 = 105.99 g/mol
Theoretically: KMnO4

What is the percent
composition of K in
KMnO4?
Homework Assignment

Snickerdoodles Homework!
EMPIRICAL FORMULAS &
MOLECULAR FORMULAS
Text 4.5 & 4.7: Page 185-193
Agenda
1.
2.
3.
Homework Review
CSI: Unknown Identification
Teacher Led Discussion


4.
Practise Problems

5.
Empirical Formula
Molecular Formula
Evidence Analysis
Homework
Learning Goals


Students should be able to calculate the empirical
formula when given the percent composition of a
sample
Students should be able to calculate the molecular
formula when give the percent composition and
molecular mass of a sample
CSI: Evidence Analysis
Empirical Formula

A formula that is derived from experimental
observations
 Not

theory
Tells us the simplest ratio that the elements are
combined in
Ethyne v. Benzene
What is the EF?
1.
C6H6
2.
C8H18
3.
WO2
4.
C2H6O2
Finding the Ratio

We need to know the percent composition by mass
 You
then convert the mass in grams to moles
 You can use the moles as the EF’s subscripts
 This

may require multiplication to get whole numbers
EX: You test a small sample and find the compound is
60% magnesium and 40% oxygen
Practise Problem

What is the EF for a compound found to have a
percent composition as follows: 21.6% sodium,
33.3% chlorine and 45.1% oxygen.
Technology

Generally to determine the percent composition
Combustion Analysers are used
 Small
sample of substance burned in a combustion
chamber
 When compound burned :
O
combines with C to make CO2
 H combines with O to make H20 vapour
 All
other elements present will convert to oxides
 Quantities of all these products precisely measures
and used to determine % composition
But...

EF does not necessarily provide correct
information about the number of atoms in a
molecule
 Does
not tell us the actual quantity of atoms in the
sample
 It tells us the simplest ratio of the atoms

Therefore we commonly need the molecular
formula
Molecular Formula

You need to know the mM
 You
then see how many multiples of the EF can fit in the
mM
 Multiply the subscripts by this number to get the MF

EX: The EF of a compound is found to be CH3 and the
mM is found to be 30.00g/mol . What is the MF?
Practise Problem

A compound with an empirical formula of C2OH4
and a molar mass of 88 grams per mole. What is
the molecular formula of this compound?
Technology
 Mass
spectrometer commonly used
 Small
sample bombarded by beam of e Causes molecules in sample to break up into charged
fragments
 Fragments are accelerated by an electrified field and
deflected by a magnetic field


Deflection will alter based on mass and charge of fragment
Deflection used to determine mM of original sample can be
detected
Another Practise Problem...

Knowing the EF is C3H4O3, what is the MF if the mM
is determined to be 176.14 g/n?
Homework