Chemistry SM-1131
Week 8 Lesson 1
Dr. Jesse Reich
Assistant Professor of Chemistry
Massachusetts Maritime Academy
Fall 2008
Class Today
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Poem
Quiz Correction
Grams, atoms, mols, avogadro’s number
“Moles,” g-> molecules, molecules  atoms, grams
molecule  gram element, mass percent, mass
percent from a chemical formula
• Take Worksheets on moles Wednesday (each
completed packet is worth 5 test points)
• Test Friday: Nomenclature and formula writing (Ionic,
molecular, acids) mole and gram conversions.
When I was one-andtwenty
• By A. E. Housman (1859-1936)
When I was one-and-twenty
I heard a wise man say,
'Give crowns and pounds and guineas
But not your heart away;
Give pearls away and rubies
But keep your fancy free.'
But I was one-and-twenty,
No use to talk to me.
When I was one-and-twenty
I heard him say again,
'The heart out of the bosom
Was never given in vain;
'Tis paid with sighs a plenty
And sold for endless rue.'
And I am two-and-twenty,
And oh, 'tis true, 'tis true.
Quiz Correction
• Put away your pens
• Take out your quizes
• Switch quizes with your neighbor
• Print your name in red ink on their quiz
• Put a big X by a wrong answer
• Put a check by a correct one
• Grade your neighbors quiz:
8-0.1(wrong answers)= quiz grade
Molecular Mass
• To figure out molecular mass you have to
know the atomic mass.
• Let’s start easily
• Ne- it exists by itself and doesn’t form
molecules. It’s mass is just the atomic mass of
Ne, which is 20.18
Simple Molecule Mass
• The mass of N2 is going to be twice the mass
of 1 atom of N
• So, if N has an atomic mass of 14.01, then N2
must have a mass of 2x14.01 or 28.02
Molecular Mass
• Ozone has the formula O3, what is it’s
molecular mass
• 1 Oxygen has a mass of 16.00
• 3x(16.00) has a mass of 48.00 amu
More Complex Masses
• Water has the formula H2O
• The molecular mass is going to be from 2H
atoms and 1 O atom, so
• 2x(1.0079) + 1x(16.00)= 18.0158amu
Sugar
• C6H12O6
• 6(12.01) + 12(1.0079) + 6(16.00)= ???
Moles
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Dozen: 12 somethings
Baker’s Dozen: 13 Somethings
A Score: 20 Somethings
Avogadro’s number: 1 mol= 6.022e23
somethings
See how it works
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A dozen atoms = 12 atoms
A baker’s dozen atoms = 13 atoms
A score of atoms = 20 atoms
A mole of atoms = 6.022e23 atoms
Moles
• It just means a big number.
• 602,200,000,000,000,000,000,000,000
• But we do this because it converts amu to
grams
Moles
• 1 mole of atoms = 6.022e23 atoms
• 2 moles of atoms= 2(6.022e23)atoms=
1.2044e24 atoms
• 3 moles of atoms = 3(6.022e23)atoms=
1.8066e24
Moles
• 1 mole of kittens = 6.022e23 kittens
• 2 moles of kittens= 2(6.022e23)kittens=
1.2044e24 kittens
• 3 moles of kittens = 3(6.022e23)kittens=
1.8066e24
Why a 6.022e23
• 1 amu = 1.66e-24 grams
• So, 1.66e-24gx6.022e23= 0.99652g which is very
similar to 1g.
• The point is that if you multiply the mass of
something in amu you can convert amu into a mass
in grams
• We don’t weigh anything in amu, but we do in grams
so this is useful.
• So, if we multiply the atomic mass of something by
1 mol it turns from amu into grams.
If you have 1 mole of N2
how much would it weigh?
• Atomic Mass of N= 14.01 amu
• Molecular Mass of N2= 28.02 amu
• 6.022e23 atoms of N2 x 28.02 amu x 1.66e-24g =
1 atom
1 amu
Which equals 28.02g.
So, 1 mol x molecular mass = # of grams
What do we do with
this?
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Chemists generally convert moles into atoms.
Atoms into Moles
Moles into grams
Grams into Moles
Moles into atoms
• 1 mole has 6.022e23 atoms in it
• 5 moles of Ne x 6.022e23 atoms = 3.011e24 atoms
1 mole
• 24.00 moles of He 6.022e23 atoms = 1.445e25 atoms
1 mole
Atoms into Moles
• You have 18.066 e23 atoms of Cu many many
moles of Cu do you have?
18.066e23 atoms x
1 mol
= 3.0000 mol
6.022e23 atoms
So
• Atoms x
1 mole
= moles
6.022e23 atoms
• Moles x 6.022e23 atoms = atoms
1 mole
Moles to grams
• We also convert moles into grams
• You can’t weigh a mole, you weigh a gram
• Moles x molecular mass in grams = grams
1 mole
Grams to Moles
• Grams -> Moles
• Xgrams x
moles = moles
Atomic mass
New Material
• “Moles,” grams molecule  gram element,
mass percent, mass percent from a chemical
formula
Mole
• 1 mole of something means 6.022e23
somethings
• Remember 1 mole of a compound = the
molecular mass in grams
Grams molecules 
moles of an element
• Grams of a molecule  moles of a molecule
 moles of an element
25 g H2O has how many moles of Oxygen in it?
25g H2O x 1 mole H2O x 1 mole O = 1.4 moles
18 gram H2O 1 moles H2O
25 g H2O has how many moles of hydrogen in it?
25g H2O x 1 mole H2O x 2 mole H = 2.8 moles
18 gram H2O 1 moles H2O
Grams molecules 
grams of an element
• Grams of a molecule  moles of a molecule
 moles of an element
25 g H2O has how many grams of Oxygen in it?
25g H2O x 1 mole H2O x 1 mole O x 16 g O = 22 grams O
18 gram H2O 1 moles H2O 1 mole O
25 g H2O has how many grams of Oxygen in it?
25g H2O x 1 mole H2O x 2 mole H x 1.0 g H = 2.8 grams H
18 gram H2O 1 moles H2O 1 mole O
Grams molecules 
grams of an element
• Grams of a molecule  moles of a molecule
 moles of an element
88 g Fe(II)Cl2 has how many grams of Fe(II) in it?
88g Fe(II)Cl2 x 1 mole Fe(II)Cl2 x 1 mole Fe(II) x 55.85 g Fe(II) = 39 grams Fe(II)
126.75 gram Fe(II)Cl2 1 moles Fe(II)Cl2 1 mole Fe(II)
88 g Fe(II)Cl2 has how many grams of Cl in it?
88g Fe(II)Cl2 x 1 mole Fe(II)Cl2 x 1 mole Cl
x 35.45 g Cl = 25 grams Cl
126.75 g Fe(II)Cl2 1 moles Fe(II)Cl2 1 mole Fe(II)
Mass Percent
• Mass percent describes how much of a
compound 1 element makes up
• A high mass percent means it makes up most
of the mass of the compound.
• A low mass percent means it makes up just a
little mass of the compound.
• Whenever we talk about a percent we mean
Little Number x 100 = percent
Big Number
Mass percent
• For Element A of compound AB
• Atomic Mass A x # of A atoms x 100 = mass percent
Molecular mass of AB
• CO2
1x(12) amu x 100 = 27% carbon
44 amu
• SF6
100xMass of 6 F atoms or 114 amu x 100=88%
Molecular Mass
146 amu
Percent Composition
• Percent Composition is when you figure out
the mass percent for every element in the
compound.
• Sodium chloride has the formula NaCl. What
is the percent composition of all elements?
• 23 amu / 58 amu x 100 = 39 % Na
• 35 amu / 58 amu x 100 = 61 % Cl
You can do this
backwards too
• Racecar is a palandrome, but it’s harder going backwards
• Cl makes up 61% of sodium chloride. What’s the formula?
1- convert % to gram
2- Write the number of grams of element A
3- Convert from grams to moles by multiplying by 1 mole per molecular mass
(in grams)
4- repeat steps 2 and 3 for every element
5- divide all number of moles by the smallest number of moles
6- reduce, double, or triple until all atoms are expressed as
whole numbers, then write the formula
Go through the steps
• 1- 61% Cl 61g Cl
• 2- 61g Cl
• 3- 61g Cl x 1 mole Cl = 1.7 moles Cl
35 g Cl
• 4- 39% Na  39g Na
• 5- 39g Na x 1 mole Na = 1.7 moles Na
• Decide which number is smaller. Divide both
numbers by 1.7 moles. You get 1 mole of Na and Cl.
So the formula must be Na1Cl1
Iron(II) Carbonate
• In class right now. Figure out the formula
(carbonate is (CO3)2-).
• Figure out the percent composition of every
element.
• Use the percent composition to back out the
formula.
Empirical Formula
• What you are actually finding is the empirical
formula. It’s the “reduced” form of a
molecular formula. For ionic compounds the
“reduced” form is always the molecular
formula. For molecular compounds they are
usually different.
Ethylene and butadiene
C2H4 and C4H8
• Perform a percent composition for both
formulas
• Compute the empirical formula for both
• How are they related? Is the empirical
formula the same thing as either molecular
formula? How would the molecular formula
of either relate to the empirical formula?
Emipirical Formula to
molecular formula
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Molecular formula = n x empirical formula
Molecular mass = n x empirical formula mass
Molecular mass/empirical formula mass = n
In a typical quiz/test problem you’ll usually
have to calculate molecular mass from a
formula, and you’ll be given either n or
empirical formula mass and have to find the
last variable.
Ethylene and butadiene
C2H4 and C4H8
• Calculate the n term for both of the above
compounds.
Wednesday
• Wednesday is a working class day
• Come to class, be prepared with the moles packets,
your book, a syllabus for homework problems, and a
calculator
• I may or may not be here on Wednesday. You are
expected to be in class. One of you will take
attendance if I’m not here. Anyone missing class
without sending me a note prior to class will be
docked a letter grade from their final grade (B+  B).
Test Friday
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You will have a test Friday
There will be formula writing questions
There will be nomenclature questions
There will be ionic vs acid vs molecular qs
There will be mol -> gram, gram -> mol, gram  mol
 atom, gram  mol  element  gram
• I will ask you for percent composition, empirical
formula, molecular formula for either CH2O &
C6H12O6, C2H2 & C6H6, or C2Cl2 & C6Cl6
• If you can’t do some of those question get help!