Converting from Grams to Moles
The gram formula mass of a compound (or element) can be defined as the mass
of one mole of the compound. As the definition suggests, it is measured in
grams/mole and is found by summing the
atomic weights
of every atom in the compound.
Atomic weights on the periodic table are given in terms of amu (atomic mass
units), but, by design, amu correspond to the gram formula mass. In other
words, a mole of a 12 amu carbon atom will weigh 12 grams.
The gram formula mass can be used as a conversion factor in stoichiometric
calculations through the following equation:
Moles = 

Gram formula mass is also known as GFM. You may also see the term gram
molecular mass, abbreviated GMM. This term is often used instead of GFM when
the substance is molecular and not ionic. However, only the terminology is
different, GMM is used in the same way as GFM. Therefore, I will use the catch
all term GFM in this study guide.
Converting between Volume of a Gas and Moles
The Ideal Gas law, discussed at length in the
Sparknote on Gases, provides a handy means of converting
between moles and a gas, provided you know certain qualities of that gas. The
Ideal Gas Law is PV = nRT, with n representing the number of moles. If we
rearrange the equation to solve for n, we get:
n = 

with
P representing pressure in atm,
V representing volume in liters,
T
representing temperature in Kelvins, and
R the gas constant, which equals .0821
Latm/molK. Given
P,
V, and
T, you can calculate the number of moles of
substance in a gas.
In those instances when a problem specifies that the calculations are to be made
at STP (Standard Temperature and Pressure; P = 1 atm, T = 273 K)), the
problem becomes even simpler. At STP, a mole of gas will always occupy 22.4 L
of volume. If you are given a volume of a gas at STP, you can calculate the
moles in that gas by calculating the volume you are given as a fraction of 22.4
L. At STP, 11.2 L of a gas will be .5 moles; 89.6 L of gas will be 4 moles.