8/14/2013
Masses of Isotopes
determined with a mass
spectrometer
Isotopes
11B
10B
• Because of the existence of isotopes, the mass of a
collection of atoms has an average value.
• Average mass = ATOMIC WEIGHT
• Boron is 20% 10B and 80% 11B. That is, 11B is 80
percent abundant on earth.
• For boron atomic weight
= 0.20 (10 amu) + 0.80 (11 amu) = 10.8000 amu
Isotopes & Atomic Weight
• Always carry the atomic weight out to four
decimal places and label it in “amu”s
• 6Li = 7.5% abundant and 7Li = 92.5%
6.9250 amu
– Atomic weight of Li = ______________
•
28Si
= 92.23%, 29Si = 4.67%, 30Si = 3.10%
– Atomic weight of Si =
28.1087 amu
______________
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8/14/2013
Another type of Atomic Weight
Problem
• What is the atomic weight of a sample of sodium
that has 345 atoms with a mass of 23 amu,
777,555
,
atoms with a mass of 22 amu and 16,525
,
atoms with a mass of 24 amu?
345(23 amu) + 777,555(22 amu) +16,525(24 amu)
(345 + 777,555 + 16,525)
= 22.0420 amu
MOLECULAR WEIGHT AND
MOLAR MASS
Molecular weight = sum of the
atomic weights of all atoms in
the molecule.
Molar mass = molecular weight
in grams/mole
THE MOLE
• A mole is a quantity of any atom, molecule
or ionic compound.
• 1 mole = 6.022
6 022 x 1023 atoms or molecules
• Atomic mass or atomic weight is the mass
of 6.022 x 1023 atoms. It is in g/mol
• Molar mass is the combined mass of a unit
of the compound and is given in g/mol
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8/14/2013
What is the molar
mass of ethanol,
C2H6O?
1 mol contains
2 mol C (12.01 g C/1 mol) = 24.02 g C
6 mol H (1.008 g H/1 mol) = 6.048 g H
1 mol O (16.00 g O/1 mol) = 16.00 g O
TOTAL = molar mass = 46.07 g/mol
Use 4 sig. figs. for molar mass
Molar Mass
How many moles of alcohol are
there in a “standard” can of beer
if there are 21.3 g of C2H6O?
(a) Molar mass of C2H6O = 46.07 g/mol
(b) Calculate moles of alcohol
21.3 g •
1 mol
= 0.462 mol
46.07 g
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8/14/2013
How many molecules of alcohol are there
in a “standard” can of beer if there are
21.3 g of C2H6O?
We know there are 0.462 mol of C2H6O.
0 462 moll •
0.462
6.022
6
0 x 10
0 23 molecules
o ecu es
1 mol
= 2.78 x 1023 molecules
How many atoms of C are there in a
“standard” can of beer if there are 21.3 g
of C2H6O?
There are 2.78 x 1023 molecules.
Each molecule contains 2 C atoms.
Therefore the number of C atoms is
Therefore,
2.78 x 1023 molecules •
2 C atoms
1 molecule
= 5.57 x 1023 C atoms
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