The Mole: Avogadro's Number and Stoichiometry

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


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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