This topic is probably the most important topic
to understand in order for you to successfully pass
your Chemistry class. You will be converting units
all semester long. So let's start with understanding the idea
of unit conversion and how conversion factors work.
Let's say you wanted to translate a phrase from English to French
but you don't speak French you only speak English
and you don't have access to google translate. You do however
have a friend who speaks both english and Spanish
and your friend knows someone who speaks both Spanish
and French. So if we wanted to go from English to French
your friend would translate the phrase from English to Spanish
and the person your friend knows would then translate the phrase from
Spanish to French. Notice how the people who knew two languages
were either in the middle or at the end of this
and the person who only knew one language was at the beginning.
This is how unit conversion works, the given unit
is what we start with and your conversion factors are in the middle and
at the end. A conversion factor has two different
units, they allow us to go from one unit to a
different unit. Notice that we always align the same
units across from each other so they can cancel out
and thus allow us to change the unit. You know where a conversion factor goes
based off of the previous unit. You will often see
the setup of unit conversion questions like this
where the point where the lines meet represents multiplication.
As you progress in this class you will see questions like this
or even longer setups again this just means there are multiplication symbols
here. The metric system and dimensional
analysis are two topics where unit conversion is used.
Both topics allow us to convert units of length, mass
volume and time. The metric system allows us to
convert units that share a common base. For example we are able to go from
milliliters to liters or from kilograms to milligrams.
The base units we commonly see are grams, meters and liters
while for dimensional analysis. We are able to convert
units that do not share a common base. For example
we are able to go from feet to inches or kilograms to pounds.
A common mistake students make with the metric system
is not knowing if they should divide by a number
or multiply. This disconnect comes from not understanding how to find the
conversion factor, so let's fix that. These numbers are
actually representing these small and large numbers, these
are written in scientific notation we just don't show the one in front.
Let's say we were asked to convert from kilograms to grams.
We would start with the given number first, then make sure to align the same
units across from each other so they can cancel out. Now this is where
most students get stuck. If you feel this way then I
recommend trying this trick of mine. Always place
the 10 raised to an exponent with the base unit
so remember our common base units are grams,
meters and liters. So since this is kilograms
and the metric system shows that kilo is 10 raised to the third power
this will go with the grams on top. Next always place a 1 with the different unit
or the unit with one of these prefixes, so a one goes with
the kilograms. This trick will stop you from guessing
and help you get the setup right each time. So our kilograms cancel and
we are left with this as our answer. Here's another example,
this time we are using multiple conversion factors.
We are asked to go from kilometers to centimeters.
Note that there is no way for us to only use one conversion factor
because there is no conversion factor that allows us to go
from kilometers to centimeters. Instead we need to have a connecting unit
meaning we need to go from kilometers back to
meters and then to centimeters. Notice that
both conversion factors have to have one unit in common in order
for this to work so our connecting unit is the meters.
We'll start with our kilometers, next we want to go from kilometers to meters
so using our trick we will place a one with the kilometers on the bottom.
Since we always want the same units across from each other so they can
cancel and the 10 to the third power which is
what kilo is for the metric system will go on top
with the meters. Now that the kilometers cancel we can go
from meters to centimeters, so we will place the meters on the
bottom so these units can cancel and the 10
to the negative second power goes with the meters,
since centi on the metric system is this and the one
goes on top with the centimeters the meters cancel
and we get our answer in centimeters. Notice
that the trick really helps you out here because you don't have to second guess
yourself. Instead of thinking do I multiply or do
I divide, instead look at where the base unit is
and where the unit with the prefix is. Moving on to dimensional analysis. In
dimensional analysis there are set conversion factors that
you will be expected to memorize and others will be provided in the
question. Here are some common conversion factors
you will see. Let's do an example, now we will start
with our given feet and now we have to pick a
conversion factor that has feet in it so the only one we see is this one this
is our first conversion factor. We'll place the same unit of feet
across from each other so they can cancel
and the inches will go on top. Next we'll find
a conversion factor that has inches in it. Luckily
this one goes from inches to centimeters which is what we are asked to find.
This is our second conversion factor. Now align the inches across from each other
so they can cancel and the centimeters will go on top
multiply everything together and this is what we
get. Remember to round to the correct number
of sig figs, we do not look at the set conversion
factors to determine the sig figs, we instead look at the numbers given in
the original question. So we would round this up to four sig figs.
The next topic that makes up the majority of your chemistry class
is stoichiometry, this is another form of unit conversion
except we are converting units that relate to elements or
chemical reactions. Like going from grams of sodium to
atoms of sodium. Conversion factors still apply here.
We just have different conversion factors that you will be required to
know how to figure out and others you need to memorize. Let's
start with the conversion factors you need to know how to figure out. Molar
mass allows us to go from grams to moles and moles to grams. We
find the molar mass by using the masses found on the
periodic table. For compounds we will add up each
individual element's molar mass together to get the
total molar mass of the compound. A mole to mole
ratio allows us to go from moles of one compound to moles of a
different compound. Note this could also be moles of one
element to another element or element to a
compound and so on. We can find the mole to mole ratio in two
different places. If given a balanced chemical equation or
a way to find the balanced chemical equation
then we use the coefficients in the chemical equation.
If there is no coefficient in front then assume it is one.
For example we can say one mole of magnesium nitride
is equal to three moles of magnesium hydroxide
or six moles of water is equal to two moles of NH3 and
so on. If there isn't enough information and you are only given a chemical
formula then you will find the mole to mole ratio by
using the subscripts in the chemical formula. For example we
could state that in one mole of C2H4 there are two moles of
carbon or four moles of hydrogen. The conversion
factors to memorize are these two. Avogadro's number allows us to go from
moles to atoms, molecules, particles and formula
units. This number always goes with these units
and the one always goes with the moles. Note
that avogadro's number can also be rounded to this,
it depends on what your teacher prefers to use.
This conversion factor is for volume and is used for gases,
so whenever a question states at STP and gives you
liters you know you can use this conversion factor which
allows us to go from moles to liters and vice versa.
Let's figure out what conversion factors to use in this question. We are asked to
find the molecules of C2H4 and we are given
45.8 grams of C2H4, so we have to think of a plan on how to
go from grams to molecules, specifically what conversion factors
will we need. If you aren't sure what to do first
notice that all these conversion factors all equal
moles, so we could start with converting our
unit of grams to moles. To do this we will use the molar mass as
our first conversion factor. This allows us to go from grams to moles,
next we need to find what conversion factor equals moles and allows us to go
to molecules. Avogadro's number allows us to do this,
so this is our second conversion factor. Let's
set this up, starting with our given unit of 45.8
grams of C2H4, since our first conversion factor is
molar mass we need to find the molar mass of C2H4
by adding up all the individual elements masses from the periodic table
and multiply any subscripts shown. Doing this we get this as our molar mass.
The grams go on the bottom and one mole goes on top since we always want to
align the units so they can cancel. Now that we have our moles and we will use
avogadro's number to convert to molecules.
The one mole goes on the bottom and molecules goes on top,
cancel out the moles and after we multiply these two numbers
and divide by the denominator we get our answer which
should be rounded to three sig figs since the given number had three sig
figs. Because this topic is the most important
topic you will see in chemistry you need to practice this in order to
successfully pass this class and you know I love making things easier
for you, simply click the link in the description to try out these practice
problems, then come right back to the next video