Fundamentals E & F:
Moles, Mass Percents, Empirical and Molecular Formulas
Atomic Mass:
How much does one atom weight?
1 amu = 1.66 x 10
24
g
atomic mass of an element is the average atomic mass for all the isotopes
in the ratio that those isotopes occur
so sample can be treated as though each C atoms weighs 12.01 amu
3000 atoms x 14.01 amu/atom = 42,030 amu (42.01 x 10
3
amu = 4.201 x 10
4
amu)
(4.201 x 10
4
) x (1.66 x 10
24
g)/1 amu = 6.97366 x 10
20
g
Number of sodium atoms in 8.56 x 10
19
amu (average atomic mass of sodium 22.99 amu)
8.56 x 10
19
amu x 1 sodium atom/22.99 amu = 3.73 x 10
18
sodium atoms
The Mole:
Allow us to count mass # of atoms when the weight is given in grams (with out using
such small units!)
One mole = 6.022 x 10
23
units (Avogadro’s number)
A dozen eggs contains 12 eggs
A mole of eggs contains 6.022 x 10
23
eggs
A mole of nitrogen contains 6.022 x 10
23
nitrogen atoms
Any element that weights a number of grams equal to the average atomic mass of
that element contains 1 mole or 6.022 x 10
23
atoms
12.01 g of carbon contain 1 mole or 6.022 x 10
23
carbon atoms
26.98 g of aluminum contain 1 mole or 6.022 x 10
23
aluminum atoms
**To have the same number of atoms in my samples I would need 12.01 g of
carbon and 26.98 g of aluminum
I have 24.56 g of sodium (22.99 amu)  how many moles? how many atoms?
How much does 1 mole weigh?
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 Winter '08
 Pomeroy
 Atom, Atomic Mass, Isotopes, Mole, Mass, Avogadro constant, Empirical and Molecular Formulas

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