hey it's professor Dave, I want to tell you about moles
what in doing chemistry we have to deal with the fact that we can't see molecules
they're just way too small. but they still obey stoichiometry
that is they react in certain proportions. these proportions have
nothing to do with mass
since every molecule has a different mass rather
they are by number as we saw in balancing equations. so to do chemistry we need a
way to talk about molecules
numerically and that number has to be so big
that it represents a number of molecules that we can see with our eyes
enter: the mole. a mole is just a number
like a dozen or a score but it is a huge number
it is Avogadro's number which is 6.022 times ten to the 23rd
that's almost a trillion trillion! what's special about this number is that it is
the number of carbon atoms
in exactly 12 grams. one carbon atom with six protons and six neutrons
weighs 12 atomic mass units so a mole
is the number of carbon atoms that weigh that same amount
but in grams which is an amount that we can see and touch
and do chemistry with. so the mole is the way to go between
atomic mass units and grams. now when we look at a chemical reaction we can
say one
A and 1 B make one AB, or we can say
a hundred A and a hundred B make a 100 AB or we can say one mole of A and one mole
of B
make one mole of AB which will probably be an amount that we can weigh on a
balance and do experiments with
every substance will have its own molar mass
which is the same as its molecular mass but expressed in grams
instead of atomic mass units. for example one water molecule
weighs 18 atomic mass units, 16 for an oxygen atom
and one for each hydrogen atom which are the average atomic masses
from the periodic table. by definition then a mole of water molecules
weighs 18 grams, about this much. to find the molar mass of a substance
you just add the atomic masses for all the atoms in the molecule
let's try one for practice
now that we understand the mole we can do
stoichiometric calculations which allow us to make predictions about a chemical
reaction
for example let's say we are performing this combustion reaction
we start with 20 grams of propane and we want to know what mass
of water we are going to produce. this is very powerful
because mathematical predictions like these are what enable us to use our
understanding of Chemistry
to create technology. so what we need to do is convert this mass of propane
into moles because knowing the mass of something tells us nothing about the
mass of something else we can expect
since every compound has a different mass but these coefficients
tell us about the numbers of molecules involved and moles
are a number so let's convert to moles. we can do this
by using the molar mass of each substance. for propane
thats 44. remember
when we convert between units we multiplied by a fraction
equal to 1. this many grams of propane does
equal one mole which is why this doesn't alter the quantity
so we can see the twenty grams of propane is actually 0.45
moles of propane. that's a number of molecules
if we have that we know how many moles of water to expect
because there is a four to one ratio. if one propane molecule
makes four water molecules then one mole of propane molecules
makes four moles of water molecules so we multiply by the stoichiometric ratio
seeing that units and names of substances cancel out
and now we have a number of moles of water to expect
exactly four times the number of moles of propane
then we can use the molar mass of water
to see what mass that number of water molecules
represents. again we want to make sure to put this unit
where it needs to go to cancel out the existing unit
in this case on the bottom. 32 grams water
is what we should get
these calculations can easily be performed all at once
just place all these conversion factors next to one another
and make sure the units cancel the way you want them to. if they do
you probably did it right. common problems of this type
will have you convert from a mass of one thing to the moles of that thing
to the moles of another thing back to grams
for that other thing. let's check comprehension
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and as always feel free to email me