Transcript Counting Atoms…

Finding the Molar Mass of Elements
amu
grams / mole
Using the periodic table, what is the molar mass of…
Aluminum 26.98 g/mol
26.98 g Al = 1 mol Al
Lead
207.20 g/mol
207.20 g Pb = 1 mol Pb
Notice, we’re still
talking about 1 mole.
Each mole has the
same number of
particles (6.02 x 1023),
just different masses!
Finding the Molar Mass of a Compound
It’s simple: Just add up its atomic masses.
Example: What is the molar mass of water?
H2O
2 H = 2 x 1.01 = 2.02 g/mol
1 oxygen = 1 x 15.99 = 15.99 g/mol
2.02 g + 15.99 g = 18.02 g/mol
molar mass of water = 18.02 g/mol
Calculate These Molar Masses:
Mg3(PO4)2
C12H22O11
262.84 g/mol
342.34 g/mol
• Molar mass is the generic term for the
mass of one mole of any substance
• Also called:
Gram Molecular Mass (molecules)
Gram Formula Mass (ionic cmpds)
Gram Atomic Mass (elements)
Percent Composition
The percentage (by mass) of each
element in a compound
Percent–means ‘parts per HUNDRED’
So, you have 100 beans.
If 18 beans are yellow – 18%
If the rest are black, what percentage of
black beans would you have?
82% black
Determining Percent Composition
The problem can be presented two ways…
1. All of the info you need is given, or
2. You have to figure out molar masses…
Either way, what you do…
Divide the mass of the element by the
total mass of the compound then multiply
by 100. That’s the percent!
This is what it looks like when a problem
gives you all the info…
You have 50g of NaCl. If you have 9g of
sodium, what is the % composition?
Mass of element
X
Total mass
9g
x
=
100
100
=
%
comp.
18%
50 g
How many grams of Cl do you have? 41 g
What percent is that?
82 %
This is what it looks like when you have to find
the molar masses from the chemical formula…
Determine the percent composition of Ammonium
Sulfate, (NH4)2SO4.
1st, find the total molar mass for the compound.
(2x14.01) + (8x1.01) + 32.07 + (4x16.00) =
132.17 g/mol
Then, find each element’s percent comp.
( 2 x 14.01) / 132.17 x 100 = 21.2% N
( 8 x 1.01) / 132.17 x 100 = 6.1% H
32.07 / 132.17 x 100 = 24.3% S
( 4 x 16.00) / 132.17
x 100 = 48.4% O
Finding Grams from Percent
You have a 90g sample of aluminum
oxide that is 28% aluminum, how many
grams of Al is that?
The question is essentially asking,
“What is 28% of 90?”
First, convert 28 % to a decimal:
28% / 100  0.28
Then, 0.28 x 90 g is the amount of Al
 25.2 g
Using Percent Composition
You have two samples of iron oxide. Determine if
they are the same compound. A 36 g sample
contains 28 g Fe. A 160 g sample contains 112 g Fe.
(If they have the same percent of iron, they must have
the same percent of oxygen and will therefore be the
same compound!)
(28 / 36) x 100 = 78%
(112 / 160) x 100 = 70%
Not the same!!!
Using Percent Composition
How many grams of oxygen can be
recovered when 50 g of KMnO4
decomposes?
1. Find the % oxygen in the compound.
•
•
Molar Mass: 158.04 g/mol
[(4 x 16) / 158.04] x 100 = 40.5% O
2. 40.5% of 50 grams is how much
oxygen can be recovered:
•
0.405 x 50 g = 20 g of oxygen
Practice Problem Using Molar Mass
How many grams are in 0.25 mol of NaCl?
1. Find the molar mass. (This will be the conversion factor.)
Na: 23.00 g/mol; Cl: 35.45 g/mol;
NaCl: 23.00 + 35.45 = 58.45 g/mol
2. Use dimensional analysis to solve.
58.45 g NaCl = 1 mol NaCl
0.25 mol NaCl 58.45 g NaCl
1 mol NaCl
=15 g NaCl
Another Practice Problem
• How many moles are in 25 g of NaCl?
1. Find the molar mass.
(This will be the conversion factor.)
Na: 23.00 g/mol; Cl: 35.45 g/mol
NaCl: 23.00 + 35.45 = 58.45 g/mol
2. Use dimensional analysis to solve.
1 mol NaCl = 58.45 g NaCl
25 g NaCl
1 mol NaCl
58.45 g NaCl
=0.43 mol NaCl
What is a MOLE?
• Abbreviated “mol”
• The amount of a substance that contains
6.02 x 1023 representative particles.
• 6.02 x 1023 is called AVOGADRO’S NUMBER.
• It is an equivalency—a conversion factor
• Just like…1 dozen = 12 pieces,
•
1 mole = 6.02 x 1023 pieces
Why 6.02 x 1023???
• Because atoms are so small, Avogadro’s
number has to be pretty big. How big?
–602,000,000,000,000,000,000,000
–Scientists use this number because…
• It is the number of particles in 12
grams of carbon-12
If you had a mole of water molecules, how
much water do you think this would be?
1 mole of water
molecules (6.02
x 1023 molecules
of water) is only
about 18 mL of
water !!
1 MOLE
of Atoms
Using Moles in Chemistry
Definition – the amount of a substance that
contains 6.02 x 1023 representative particles.
What is a representative particle?
• It is whatever you are counting:
– Molecules
– Formula units
– Atoms
– Ions
A Typical Question: How many molecules of CO2
are there in 4.56 moles of CO2 ?
• Our representative particle in this question is a
molecule—it is what we are counting.
• We can use dimensional analysis:
1 mol CO2 = 6.02 x 1023 molecules CO2
4.56 mol CO2
6.02 x 1023 molecules CO2
1 mol CO2
(4.56) 6.02 x 1023 molecules CO2=
2.75 x 1024 molecules CO2
Moving On…
• Now that we can find how many
representative particles are in a given
number of moles, and…
• How many moles are represented by a
given number of representative particles,
• We need to learn about the mass of a
mole.
The MOLAR MASS!!
Measuring the Mass of Moles
• Remember atomic mass?
• The amu (atomic mass unit) is one twelfth
the mass of a carbon-12 atom.
• Since the mole is the number of atoms in
12 grams of carbon-12…
•The decimal number on the periodic
table (average atomic mass) is ALSO the
mass of 1 mole of those atoms in grams.