Secs 3.3-3.4: Formula Weights
Formula Weights and Molecular Weights
Formula weight = sum of atomic weights in a substance's
chemical formula
Formula weights are given in units of amu
E.g., find the FW of HNO3
If the chemical formula of a substance is the same as the
molecular formula of a substance, then formula weight is
often called molecular weight (MW)
When doesn't MW = FW?
Examples?
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Given the chemical formula of a substance, how do we
find the percentage composition of each element in the
compound?
e.g., calculate the percentage by mass of each
element in NH4Br
The Mole
Even a very small sample of water (e.g., a teaspoon)
contains an enormous amount ( ~2 x 1023 ) of H2O
molecules!
The mole (mol) is the unit we use in chemistry to describe
very large quantities of atoms, etc.
1 mol is defined as the amount of matter which contains
as many objects (atoms, ions, doughnuts, etc.) as the
number of atoms in exactly 12 g of 12C.
Huh?
50
How many C atoms are in exactly 12 g of 12C?
There are 6.0221367 x 1023 C atoms in exactly 12g of 12C
6.0221367 x 1023 is known as Avogadro's number (we use
6.02 x 1023)
Avogadro’s number is the number of things per mol –
something
it has units of
mol
12
23
E.g., for 1 mol C atoms, we have 6.02 x 10
12
Catoms
mol

For 1 mol OH- ions, we have 6.02 x 1023
OH ions
mol
Look at the units of Avogadro’s number – it is a conversion
factor between mol and atoms/molecules/ions!!
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E.g., how many Ar atoms are present in 0.053 mol Ar?
E.g., how many mol of oxygen atoms are present in 1 mol
of HNO3 ? How many oxygen atoms is this?
Molar mass
The mass of a single atom of an element (in amu) is
numerically equal to the mass (in g) of 1 mol of atoms of
that element
e.g., 1 12C atom weighs 12 amu; 1 mol 12C atoms weighs 12
g
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The mass (in g) of 1 mol of a substance is called the molar
mass (MM) of the substance
The molar mass (in g) of any substance is always equal to
its formula weight (in amu)
FW or MW has units of amu
MM has units of g/mol
e.g., what is the molar mass of nitric acid?
g
; it is a conversion factor
mol
from gmol or mol  g!!
Molar mass has units of
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Converting masses, moles, and numbers of particles
1. Grams moles
How many moles of Mg(NO3)2 are there in 5.20 g of this
substance?
2. Moles  atoms
How many N atoms are there in 3.75 mg of Mg(NO3)2?
3. Moles  grams
What is the mass in g of 0.0885 mol Mg(NO3)2?
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Problems du Jour
Calculate the mass, in g, of 2.98 x 1021 Ar atoms.
Calculate the number of molecules in a 50.0 mg tablet of
acetaminophen, C8H9O2N.
How many mol of copper (II) nitrate are present in 1.25 g
of this substance? How many N atoms?
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