Title: Atomic Weight, Molecular Weight, Formula Weight and Molar Mass
1Atomic Weight, Molecular Weight, Formula Weight
and Molar Mass
loosely based on Chap 3 Sec 1of Brady Senese
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2Atomic Weight or Atomic Mass
- Examine the periodic table. What is the atomic
weight (AW) of carbon?
The atomic weight of C is 12.01 amu/atomor
12.01 amu/atom
It means that each C atom weighs12.01 u (or
12.01 amu). (In this class you should make it a
habit to look up AW to 4 sig. fig.)
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3Molecular Weightor Molecular Mass
The AW of C is 12.01 amu/atom The AW of O is
16.00 amu/atom. Therefore, the mass of one
molecule of CO2 would be 12.01 2(16.00) 44.01
amu/molecule. (Always round to 4 sig. fig.) We
call this the molecular weight (MW) or molecular
mass. What is the MW of methane (CH4)?
Ans. 4.032 amu/molecule
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4Formula Mass
If we add up the atomic mass of Na and Cl we get
22.99 35.45 58.44. What is wrong with
saying the molecular weight of NaCl is 58.44
amu/molecule?
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5Formula Mass
For molecular substances, the smallest unit is
the molecule. For ionic substances, the smallest
unit is the "formula unit" abbreviated f.u. e.g.
If you pluck out the smallest unit out of water,
you would have a water molecule.
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6Formula Mass
When you pluck out the smallest unit of table
salt you would have a formula unit of NaCl.
NaCl-
NaCl?NaCl?NaCl? Cl?NaCl?NaCl?Na NaCl?NaCl?
NaCl?
We often simply write NaCl, but it is understood
that Na is Na and Cl is Cl-.
There are no molecules in an ionic compound!
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7AW, MW and FW
- Summary
- Atomic Weight (AW) mass of one atom
- (for atoms)
- in units of amu/atom
- Molecular Weight (MW) mass of one molecule
- (for molecular substances)
- in units of amu/molecule
- Formula Weight (FW) mass of one f.u.
- (for ionic compounds)
- in units of amu/f.u.
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8Calculations of AW, MW and FW
- Calculate the formula weight of
- (and give the proper units)
- cobalt(III) sulfide
- ammonium sulfate
- Calculate the molecular weight of
- (CH3CH2)3N
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9- Calculate the formula weight of
- Na2SO4?2H2O
- This is known as a hydrate.
- The dot in the formula does NOT mean you multiply
anything. - The dot means there are 2 water molecules
attached to each f.u. of Na2SO4. - What is its formula weight?
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10The Mole Concept
One mole is defined as the number of atoms in
exactly 12 g of C-12 atoms. Remember that C-12
refers to the isotope.
exactly 12 g C
6.022x1023 C atoms
It turns out that there are 6.022x1023 C atoms in
those 12 grams. So. 1 mole 6.022x1023(not an
exact number, but in 4 sig. fig.)
MEMORIZE THIS NOW!
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11The Mole Concept
- 1 mole 6.022x1023
- This number is known as
- Avogadro's number
- Note Mole is abbreviated mol
- NOT m (which stands for meters)
- Note It is a NUMBER.
- It is NOT a mass.
- It does NOT have the unit of grams.
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12The Mole Concept
- We all know
- 1 dozen eggs 12 eggs
- 1 dozen atoms 12 atoms
- In the same way, we use the word "mole"
- 1 mole eggs 6.022x1023 eggs
- Obviously that is a VERY big number of eggs.
There would be no occasion to be talking about a
mole of eggs. - When do you we use "moles?"
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13The Mole Concept
1 mol atoms 6.022x1023 atoms 1 mol molecules
6.022x1023 molecules 1 mol electrons 6.022x1023
electrons The "mole" is not a mass itself, just a
number of items. Why do we bother with such a
strange number? Let us take a look at the
periodic table.
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14The Mole Concept
The atom is a very, very small particle. You can
fit 7000 atoms on the dot of an i. It is too
small to see with the naked eye, and too small to
hold in your hand or weigh on the balance. We
must use a much larger quantity of atoms a mole
of atoms.
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15The Mole Concept
From the periodic table, we can get the atomic
weight for Ag. This mass can be given in two
units 107.9 amu per atom or 107.9 g per mole of
atoms
One atom of silver weighs 107.9 amu, which is
0.0000000000000000000001079 gram Get the picture?
You cannot weigh a single atom of silver, but you
CAN weigh a mole of Ag atoms. 107.9 g is about
3.8 ounces.
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16The Mole Concept
From the periodic table we see that Ag is 107.9
g/mol Cu is 63.55 g/mol What does 107.87 g of Ag
have in common with 63.55 g of Cu? They both
contain the same number of atoms 6.022x1023 Ag
atoms and 6.022x1023 Cu atoms, in other words, 1
mole Ag and 1 mol Cu.
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17The Mole Concept
Earlier we were talking about the AW, MW and FW.
Now we are going to add one more. MM molar
mass Whenever you see "molar" you should think
"per mole." Molar mass mass per mole (for 1
mol) with units of
g/mol KNOW THIS WELL! SEE THIS IN YOUR
DREAMS.AND NIGHTMARES!
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18Molar Mass (MM)
The atomic mass from the periodic table can be
used with two units amu/atom or g/mole (or g
mol-1) REMEMBER Whenever you see a fractional
unit (such as amu/atom, g/mol, g/mL) you should
think "conversion unit"! The g/mol gives you the
link between MASS AND of MOLES.
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19Molar Mass (MM)
The FW ofCo2(SO4)3 is 406.7 amu/f.u. Its MM
406.7 g/mol 2(58.93)3(32.07)12(16.00)
406.7 We had calculated the MW of (CH3CH2)3N
to be 101.2 amu/molecule Its MM 101.2
g/mol 2x3C 5x3H N 6(12.01) 15(1.008)
14.01 101.2
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20Molar Mass (MM)
Note that we no longer need to distinguish
between f.u. and molecules in the units of
MM. It is understood that for Co2(SO4)3 its MM
406.7 g/mol of f.u. and for (CH3CH2)3N its MM
101.2 g/mol of molecules.
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21Use of MM in Dimensional Analysis
Conversion factors from MM e.g. MM (N)
14.01 g/mol
e.g. MM (HCl) 36.45 g/mol
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22Use of MM in Dimensional Analysis
When do we use MM?
Whenever you see a fractional unit (such as g/mol
or g mol-1) you should think "conversion
factor"! MM is the link between mass moles
mass moles of particles
moles of atoms moles of f.u. moles of
molecules e.g. mol Cu mol CuCl2 mol PBr3
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23Use of MM
- Example 3.1 p.89
- How many moles of sulfur are there in a 23.5 g
sample of sulfur? - Example 3.2 p.89
- We need 0.254 mol of iron(III) chloride for a
certain experiment. How many grams do we need to
weigh? - Do Practice Exercises 1 2 on p.90
24Use of Avogadro's Number in Dimensional Analysis
Conversion factors from Avogadro's number
Note that X can be anyTHING, such as CO2, K2SO4,
electrons, Na, etc. as long as it is EXACTLY
the same on top as on the bottom. BUT it cannot
be a unit, such as g or mL.
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25Use of Avogadro's Number in Dimensional Analysis
You use Avogadro's number ONLY if you have to go
between the macroscopic and the particulate
(sub-microscopic) levels.
particulate level macrosopic level
(conceptual, (observable) non-observable)
atoms moles of atoms f.u. moles of
f.u. molecules moles of molecules amu g, mg, lbs,
oz
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26Use of Avogadro's Number
- Example 3.3 p.90
- How many atoms of copper are there in a piece of
pure copper wire that weighs 14.3g? - Example 3.4 p.91
- What is the mass in grams of one molecule of
carbon tetrachloride (CCl4)? - Example
- What is the mass in grams of 2.01 mole of NaCl?
27Use of Avogadro's Number
- Example
- How many anions are in 7.82 g of MgCl2?
- Example
- What is the weight in grams of 158 molecules of
CO2? - REMEMBER! ASK YOURSELF
- DO I NEED AVOGADRO'S NUMBER IN THIS PROBLEM?
- Do Practice Exercises 3 4 on p.91