Formula Weight (Molar Mass)

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
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
Avagadro’s Number
The quantity of matter containing Avagadro’s Number of
particles, such as atoms, molecules or formula units, is
known as a:
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
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
• 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.
• 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
• 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
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
• 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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