# Science3

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Mass Relationships in Chemical Reactions:
By definition:
1 atom 12C “weighs” 12 amu
On this scale
1H = 1.008 amu
16O = 16.00 amu
Atomic mass is the mass of an atom in atomic mass units (amu)
Micro World
atoms & molecules
Macro World
grams
The average atomic mass is the weighted
average of all of the naturally occurring
isotopes of the element.
Example
Copper, a metal known since ancient times, is used in electrical cables and pennies, among
other things.
The atomic masses of its two stable isotopes, (69.09 percent) and (30.91 percent),
are 62.93 amu and 64.9278 amu, respectively.
Calculate the average atomic mass of copper. The relative abundances are given in
parentheses.
Strategy
Each isotope contributes to the average atomic mass based on its relative abundance.
Multiplying the mass of an isotope by its fractional abundance (not percent) will give the
contribution to the average atomic mass of that particular isotope.
Solution
First the percents are converted to fractions:
69.09 percent to 69.09/100 or 0.6909
30.91 percent to 30.91/100 or 0.3091.
We find the contribution to the average atomic mass for each isotope, then add the contributions
together to obtain the average atomic mass.

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(0.6909) (62.93 amu) + (0.3091) (64.9278 amu) = 63.55 amu
Check
The average atomic mass should be between the two isotopic masses; therefore, the answer is
reasonable.
Note that because there are more than isotopes, the average atomic mass is closer
to 62.93 amu than to 64.9278 amu.
Average atomic mass (63.55)
The mole (mol) is the amount of a substance that
contains as many elementary entities as there
are atoms in exactly 12.00 grams of 12C
1 mol = NA = 6.0221415 x 1023
Dozen = 12
Pair = 2
The Mole (mol): A unit to count numbers of particles
0
Molar mass is the mass of 1 mole of in grams
eggs
shoes
marbles
atoms
1 mole 12C atoms = 6.022 x 1023 atoms = 12.00 g
1 12C atom = 12.00 amu
1 mole 12C atoms = 12.00 g 12C
1 mole lithium atoms = 6.941 g of Li
For any element
atomic mass (amu) = molar mass (grams)
One Mole of:
C
S
Cu
Fe
Hg
0
1 amu = 1.66 x 10-24 g or 1 g = 6.022 x 1023 amu

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Mass Relationships in Chemical Reactions: By definition: 1 atom 12C “weighs” 12 amu On this scale 1H = 1.008 amu 16O = 16.00 amu Atomic mass is the mass of an atom in atomic mass units (amu) Micro World atoms & molecules Macro World grams The average atomic mass is the weighted average of all of the naturally occurring isotopes of the element. Example Copper, a metal known since ancient other things. times, is used in electrical cables and pennies, among The atomic masses of its two stable isotopes, are 62.93 amu and 64.9278 amu, respectively. (69.09 percent) and Calculate the average atomic mass of copper. The relative abundances are parentheses. (30.91 percent), given in Strategy Each isotope contributes to the average atomic mass based on its relative abundance. Multiplying the mass of an isotope by its fractional abundance (not percent) will give the contribution to the average atomic mass of that particular isotope. Solution First the percents are converted to fractions: 69.09 percent to 69.09/100 or 0.6909 30.91 percent to 30.91/100 or 0.3091. We find the contribution to the average atomic mass for each isotope, then add the contributions together to obtain the average atomic mass. (0.6909) (62.93 amu) + (0.3091) (64.9278 amu) = 63.55 amu Check The average atomic mass should be between the two isotopic masses; therefore, the answer is reasonable. Note that because there are more to 62.93 amu than to 64.9278 amu. than isotopes, the average atomic mass is c ...
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I was struggling with this subject, and this helped me a ton!

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