Counting Atoms
How do we really know how many atoms we have in
a sample of a certain substance? Is it possible to count
each individual atom?
Of, course not… that’s why we have…
THE MOLE
Chapter 3
Counting Atoms…
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Mole—the SI base unit used to measure the amount
of a substance whose number of particles is the
same as the number of atoms in 12g of C-12
¤ Instead
of asking for 12 eggs, you ask for a dozen
eggs are ½ dozen
¤ Etc.
Counting Atoms
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¤6
Molar Mass
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Molar mass—the mass in grams of 1 mol of a
substance
Expressed in g/mol
The mass in grams of 1 mol of an element is
numerically equal to the element's atomic mass from
the periodic table in amu.
1 mole = 6.022x1023
Avogadro’s number – the number of atoms in
exactly one mole of a pure substance
The mass of one mole of a pure substance is called
the molar mass of that substance.
Comparing Molar Masses
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One mole of Pb has a mass of 207.2 g
One mole of Al has a mass of 26.98 g
Both have 6.022 x 1023 atoms!
* The number of atoms the same, but the masses
differ!
Converting moles to mass
and mass to moles
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Gather information
Plan your work (conversion factors)
Calculate
Verify your results
Example
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What is the mass in grams of 1.10 mol of uranium?
Solution
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Information:
¤ Amount
of U = 1.10 mol
of U = ?
¤ Molar mass of U = 238.03 g/mol
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Calculate:
¤ 1.10
¤ Mass
mol U
238.03 g U
1 mol U
Plan:
¤ Factorà
¤ 1.10
238.03 g U = 1 mol U
mol U = ? g U
Example 2
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How many moles are there in 18 g Be?
= 261.83 g U
Solution
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Info:
¤ Moles
of Be = ?
of Be = 18 g
¤ Molar mass of Be = 9.01 g/mol
¤ Mass
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Plan:
¤ Factor
à 1 mol Be = 9.01 g Be
Avagadro’s Number
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Calculate:
¤ 18
g Be 1 mol Be
9.01 g Be
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Amedeo Avagadro di Quaregna (1776-1856)
determined the amount of particles in a substance.
6.0221367 x 1023
The number of atoms, molecules, formula units,
particles, green peas, or baseballs in 1 mol
= 1.99 mol Be
Particle/Mole Conversions
More examples
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Use Avogadro’s Number to convert from/to particles
to moles.
EX: How many Li molecules are there in 0.75 mole of
Li?
0.75 mol Li
6.022 x 1023 molec. of Li
1 mol Li
= 4.52 x 1023 molecules Li
Mass of Element (g)
Molar Mass
Mole of Element (mol)
1 mol = 6.022x1023 atoms
Atoms of Element
How many mol of Au are there in 7.5 x 1024 atoms
of Au?