Formula Weight (Molar Mass)

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Mass Measurements
In Chemistry
Calculating Formula Weight
• I CAN calculate the formula
weight (molar mass) of a
compound from it’s formula.
We’ve talked about the mass of atoms in terms of
one or two….but these masses are very small.
To be practical for laboratory work, a standard of
measure had to be derived so that mass
quantities could be easily measured.
To that end, scientists created a unit called the
ATOMIC MASS UNIT used to measure the mass of
subatomic particles.
The mass of a proton or neutron was assigned a value of
1 atomic mass unit (amu).
To create a system of practical measurement, scientists
used the atom CARBON-12 [6 P+ and 6 No] and
assigned it a mass of 12 AMU.
Then they calculated the number of atoms required to
make a mass of 12 grams of Carbon-12.
They found it took 6.022 x 1023 atoms of C-12 to have a
mass of 12 grams. In turn, each element’s atomic mass
required 6.022 x 1023 atoms to equal the same mass in
grams.
FOR EXAMPLE
1 atom of Oxygen 16 weighs 16 AMU .
6.022 x 1023 atoms of O-16 weighs 16 grams!
This number became very important in chemistry is known
as:
Avagadro’s Number
The quantity of matter containing Avagadro’s Number of
particles, such as atoms, molecules or formula units, is
known as a:
Mole
When working in a lab setting, the MOLE is used to determine
quantities of chemicals.
Mass Calculations and
Chemical Formulas
• Chemical Formulas represent the ELEMENTS
and the NUMBER OF ATOMS of each that
compose a compound.
– Subscripts in formulas relate to the element
immediately IN FRONT of them.
– If a formula contains a PARENTHESIS followed by a
subscript, the subscript multiplies ALL elements
inside.
• EXAMPLE
Ca3(PO4)2
3 atoms Ca
2 atoms P
8 atoms O
Calculating Formula Weight (Molar Mass)
• Formula Weight (Molar Mass) of an element or
compound is the MASS in GRAMS of ONE MOLE
of the element or compound.
• For an element, the FW is the ATOMIC MASS
from the periodic table, or for a particular
ISOTOPE.
• For a compound, the FW is found by counting the
number of each kind of atom.
• Multiply the number of atoms X the Atomic
Mass Number of the element (from the PT).
• Add the results and round to significant figures.
Example
• What is the formula weight of Calcium Carbonate?
• CaCO3
From Formula
1 mole Ca X
1 mole C X
3 moles O X
From Periodic Table
40.08 g
12.011 g
15.9994 g
=
40.08 g
=
12.011 g
= + 47.9982 g
________________
=
=
100.0892 g
100.09 g/mole
Hydrates
• Certain ionic compounds trap water molecules in
their structure when they form.
• These are known as HYDRATES.
• Hydrates are written in this manner:
• Compound Formula Dot Number of Water Molecules
•
CuSO4 5H2O
• Hydrates are named as:
• Compound Name, a Prefix for the number of water molecules,
then the word Hydrate.
• When calculating the formula weight of a hydrate, just include
the MASS OF THE NUMBER OF WATER MOLECULES in the total.
Practice Problem
• What would be the formula weight of
Magnesium Chlorate Pentahydrate?
• Formula = Mg(ClO3)25H2O
• 1 mole Mg x 24.305 g = 24.305 g
• 2 moles Cl x 35.453 g = 70.906 g
• 6 moles O x 15.999 g = 95.994 g
• 10 moles H x 1.0079 g = 10.079 g
• 5 moles O x 15.999 g = 79.995 g
•
281.279 g/mole
PRACTICE PROBLEMS
• Complete the following FORMULA WEIGHT (MOLAR MASS)
calculations ON YOUR OWN PAPER!
• 1. Find the FORMULA WEIGHT of Aluminum Sulfate, Al2(SO4)3.
• 2. What is the MOLAR MASS of Glucose, C6H12O6?
• 3. Find the FORMULA WEIGHT of:
Sodium Potassium Permanganate Tetrahydrate
NaK(MnO4)2  4H2O
• 4. What is the MOLAR MASS of Ammonium Phosphate Dihydrate.

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