Section 3.4
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Pair = 2
Dozen = 12
Ream = 500
Mole = 6.02 x 1023
1 mole is equal to
602,000,000,000,000,000,000,000 of
something
(this number is called Avogadro’s constant)
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The SI base unit that describes the amount of
a substance
1 mole of popcorn would cover the entire
United States and form a pile 310 miles high
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Atoms are tiny and it takes a LOT of them to
make up a reasonable amount of mass
To understand a reaction, you need to know
what AMOUNT of atoms are combining; not
necessarily their masses. (This comes later)
Your friends will be impressed by your ability
to confuse them.
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Atomic number 12
Magnesium
Symbol: Mg
Atomic mass: 24.3050
◦ Means that for the average Magnesium atom, there
are about 24 protons and neutrons in the nucleus
◦ Also means that 1 mole of Magnesium atoms has a
mass of 24.3050 grams. For that reason, atomic
mass is often called “molar mass”
◦ 2.0 moles of Magnesium has a mass of about 49 g.
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Atomic number: 78
Atomic name: Platinum
Symbol: Pt
Atomic mass (molar mass): 195.078
About how many protons and neutrons are in
the nucleus?
One mole of Platinum atoms has about how
much mass?
How much mass does 3 moles of Platinum
have?
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Mass: m (measured in grams)
Moles: n (multiples of 6.02 x 1023)
Atomic mass = (mass) / (# of moles)
Atomic mass = m / n
◦ Measured in “amu”
◦ Measured in g/mol
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What mass does 2.5 moles of carbon have?
◦ Step 1: What is the atomic mass of carbon?
 12.0107 g/mol
◦ Step 2: What is my value for m?
 m = unknown (that’s what I’m solving for!)
◦ Step 3: What is my value for n?
 n = 2.5 mol
◦ Step 4: Solve for the variable I want
 Atomic mass = m / n
 m = n * (Atomic mass)
◦ Step 5: Plug in your answer
 m = 2.5 mol * 12.0107 g/mol
 m = 30.02675
◦ Step 6: Round to the correct # of significant digits
 m = 30. g
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How many moles of Neon atoms are in 50.0
grams of Neon?
◦ Step 1: What is the atomic mass of Neon?
 20.1797 g/mol
◦ Step 2: What is my value for m?
 m = 50.0 g
◦ Step 3: What is my value for n?
 n = unknown (that’s what I’m solving for!)
◦ Step 4: Solve for the variable I want
 Atomic mass = m / n
 n = m / (Atomic mass)
◦ Step 5: Plug in your answer
 n = 50.0 g / 20.1797 g/mol
 n = 2.4777375
◦ Step 6: Round to the correct # of significant digits
 n = 2.48 mol
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What are the two units we can use to measure
atomic mass?
◦ amu
◦ grams per mole (g/mol)
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Carbon has an atomic mass of 12 amu. How
much mass does 4 moles of carbon have?
◦ 48 g
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What formula do we use to convert between
mass and amount?
◦ Atomic mass = m / n