Stoichiometry is a complicated sounding word but if we break it down we see that it isn't
too bad. The first part stoichio is from the Greek word for element. The second part metry
is from the Greek word for measurement. So stoichiometry is about measuring elements
and ultimately what you're gonna to see it's about balancing chemical equations. To give
a very simple example imagine that we take one atom of element A and one atom of element
B and imagine further that these two atoms react and form the diatomic molecule AB. What
we have said in words is a chemical equation. We started with one A and one B as reactants
and we ended up producing one molecule that contained A and B. So neither A nor B was
lost and importantly we didn't end up with any extra A's or extra B's. This description
is consistent with the law of conservation of matter which as you learned states that
matter is neither created nor destroyed during the course of a chemical reaction. So I hope
you're starting to see that stoichiometry is essentially a tool a tool that we can use
to help us organize chemical information and express chemical reactions in meaningful ways,
ways that are consistent with the laws of nature. As you proceed in your study of this
unit you will notice that fairly soon we change our description of chemical reactions from
one atom of element A combines with one atom of element B to one mole of A combines with
one mole of B. So why are we changing our description and what is a mole? We change
our description because atoms and molecules are so incredibly small that we have resorted
to measuring their masses as a means of counting or or helping us keep track of how many atoms
and molecules we have. As far as a mole a mole is simply a number. It's a pretty large
number it's it's about 6.022×10 to the 23rd and we can connect this concept with something
from a an everyday experience where someone might walk into a room holding a box and saying
"hey I have a dozen doughnuts in this box." Immediately you would know that that person
has a box that contains 12 donuts because you know that one dozen equals 12. Similarly
I could hold up a sample of water and say "this sample of water has a mole of water
molecules" and so you could say to yourself that sample must contain about 6.022×10 to
the 23rd molecules of water. Now you might be wondering to yourself what what size container
would I need to hold a mole of water molecules? To help you think about this we're going to
imagine that we take a beaker a 25 mL beaker and we place that beaker on an electronic
balance. We zero out the balance and then we add water to the beaker until the balance
reads 18 grams. When we do this we can say we have a mole of water molecules. So 18 grams
of water is equal to a about a mole of water molecules. Now why this is so is one of the
important things you're gonna be learning as you study this unit but to get you started
in thinking about this we're gonna introduce another concept and that concept is the Atomic
Mass Unit. Now by international agreement a carbon 12 atom has an atomic mass of 12
atomic mass units. A carbon 12 atom is a particular type of carbon. It's it's what we call an
isotope of carbon. It's the most abundant isotope of carbon. It has six protons and
six neutrons. Now because 1 gram is equal to 6.022×10 to the 23rd atomic mass units
it follows that 12 grams of carbon 12 atoms is equal to one mole of carbon 12 atoms. If
we look at a periodic table however we'll notice that the atomic mass of carbon is listed
as 12.011 atomic mass units instead of 12.000 so why is that? The reason why is because
not all naturally occurring carbon atoms are the carbon 12 isotope. Some, about 1%, are
the carbon 13 isotope where we have six protons and seven neutrons so the atomic mass of carbon
is a weighted average. It reflects the isotopic abundance of carbon that we find in nature.
This line of reasoning also holds for the other elements in the periodic table. For
our final example in our introduction to this unit we're gonna think about concentration.
In particular we're gonna think about molar concentration. Now molar concentration implies
moles per liter. To illustrate we can add potassium chloride to water and create a potassium
chloride solution. If we have a two molar potassium chloride solution that means that
in one liter it would contain two moles of dissolved potassium chloride. Now we can use
that concentrated solution of potassium chloride to make other solutions that are less concentrated.
We call those other solutions working solutions. We use an equation to help us figure out how
much of the concentrated stock solution we should use. That equation is M1V1 is equal
to M2V2 so this practical example is our final introduction to the major topic of this next
unit stoichiometry.