Tuesday, April 10
1) Review Mole Highway and take up Moles Worksheet
2) Mole Quiz
3) Percentage Composition Note
4) Practice Percentage Composition
Apr 10­9:11 AM
Moles Worksheet
2) How many moles in 34g of Cu(OH)2
3) How many moles in 2.45 x 1023 molecules of CH4
4) How many grams in 3.4 x 1024 molecules of NH3
Apr 10­8:37 AM
1
http://www.bcscta.ca/resources/hebden/chem/Coin%20Compositions.pdf
0
Uses of Percentage Composition
Percentage composition of natural substances can be
determined and used to identify the chemical formula of a
compound. Once chemists know the chemical formula, they
can often produce the compound in a lab.
Apr 9­9:47 PM
Percentage Composition
Law of Definite Proportions
Elements in a chemical compound are always present in the same proportions by mass.
ex. for water 2 parts H for every 1 part O
Percentage Composition
The relative mass of each element in a compound
ex. Vanillin, C8H8O3 , has a percentage composition of 63.1% C, 5.3% H and 31.6% O
use when
We will use 2 ways to find the mass % of each element you know
the masses
in a compound.
1) Using the mass of the compound and each element
mass % = mass of the element x100%
mass of the compound
2) Using the chemical formula of the compound
mass % = (# of element in cpd) x Melement x100%
Mcompound
use when you
know the
chemical
formula
Apr 9­4:26 PM
2
p. 201 #3
A sample of a compound is analyzed and found to contain carbon, hydrogen and oxygen. What is the percent composition of the compound?
msample = 650 mg We know the mass % = mass of the element x100%
mass of the compound
masses
mC = 257 mg
mH= 50.4 mg
mass % C = mass % H = mass % O = The percentage composition of the compound is
Apr 9­4:31 PM
p.204 #6
Sulfuric acid is an important acid in labs and industries. Determine the percentage composition of sulfuric acid.
H2SO4
we know the chemical formula
mass % H = mass % S =
mass % O = The percentage composition of sulfuric acid is
Apr 9­5:00 PM
3
Homework
Read p.197­201
p.201 #1,2
p.204 #5ab,7 p.202 Thought Lab­ % by number vs. % by mass p.205 #1 p.206 #5
Be able to describe some uses of percentage composition.
Reminders
Mole Assignment due Wednesday
Mass, Moles, Particles Lab Activity due Thursday
STSE Presentations start Friday
Apr 9­5:28 PM
4