Transcript Chemical Calculations: Formula Masses, Moles, and Chemical

Chemical Calculations: Formula Masses,
Moles, and Chemical Equations
Formula Mass

Sum of all of the atomic masses in the
formula of a substance

Use Atomic Mass Units (AMU)
1 AMU = mass of 12C / 12
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Where do you find the atomic masses?
(on the Periodic Table)
(Sometimes referred to as formula weight or
molecular weight)
Determine the Formula Mass

Remember – atomic masses are based
upon measurement
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H2O

H2SO4
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(NH2)2CO
The Mole
The Mole

The unit of “amount” in chemistry
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6.02 x 1023 of anything
(Avogadro’s Number)


The number of Carbon atoms in 12
gram of 12C
Mole Calculations
6.02 x 1023 objects
1 mole
1 mole
6.02 x 1023 objects
How many molecules are in 4.7 moles of aspirin?
If you have 7.248 x 1024 atoms of carbon, how many
moles of carbon atoms do you have?
If you have 17.4 moles of H20, how many moles of
hydrogen do you have?
Molar Mass

The mass of one mole of a substance
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Same as formula mass except change AMU
to g/mole
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Determine the molar mass of:
H2O
NH3
Ca3(PO4)2
Molar mass is a CONVERSION FACTOR

Allows conversion from moles to grams and from
grams to moles.

Plus…. it allows you to weigh out grams rather than
count out moles.
Molar Mass Conversions
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How many moles of water do you have
if you have 32.062 grams of water?
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If you need 2.0 moles of NH3 for an
experiment, how many grams do you
need to weigh out?
Chemical Equations

A way of accurately communicating what
occurs during a chemical process

Identifies the reactants (starting substances)
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Identifies the products (final substances)

Takes into account the fact that mass is
conserved and atoms are not created or
destroyed.
Combustion of Hydrogen
H2
+
O2
H2O
Does this accurately identify the
reactants and product?
Is there conservation of matter? (same
number of each type of atom on both
sides)
Balancing equations

Adding numbers to one or both sides
of an equation to ensure equal number
of each type of atom?
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What numbers can we change?

Subscripts?
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Coefficients?
Method for balancing an equation
1) Make sure reactants and products are correct
2) Examine to find out which elements are not
balanced. Pick one to balance by changing
coefficients.

Try to pick one that is only found in one molecule
on each side of the equation.
3) Pick a second element. Sometimes it helps to pick
one that already has a new coefficient on the other
side.
4) Continue the process until the equation is
balanced
5) Check your result by adding up all atoms on
both sides of the equation
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Hints:
Sometimes lone elements can be used last of
all to “clean up with”
Sometimes you can see multiples that can be
used
X2
Y3
A “3” works here
A “2” works here

More hints:
Sometimes you can use fractions to help
3/2 X2
X3
But… the final balanced equation must have
whole number coefficients in their lowest
whole number ratios
3
X2
2 X3
What do the coefficients of a balanced
equation represent?
2 H2
+
O2
2 H2 O

Grams of each substance? No

Molecules of each substance? Yes

Moles of each substance? Yes
Stoichiometry

Converting amounts of one substance into amounts
of another substance in a balanced chemical
equation.
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Mole/Mole ratio
Uses equation coefficients and allows you to convert
moles of one substance into moles of any other
substance in the equation

Molar Mass
Allows you to convert grams of a substance
into moles of a substance and vice versa.
Simplest stoichiometric conversions

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Moles of A into Moles of B
Uses mole ratio (coefficients of balanced
equation)
Next simplest conversion

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Grams of A to Moles of B
Uses molar mass of A and then mole ratio of
A to B
Finally… converting Grams of A to
Grams of B
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Grams of A to Moles of A (molar mass of A)
Moles of A to Moles of B (mole ratio)
Moles of B to Grams of B (molar mass of B)
Lets try a bunch of these….
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Make sure you have a balanced equation
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Use the molar mass of a substance to
convert moles of it to grams of it and vice
versa.
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Use the mole ratio (coefficients) to convert
moles of one substance to moles of another
substance