hello and welcome to the chemistry
solution this tutorial is on molarity
molarity is the unit of solution
concentration and can be calculated by
dividing the moles of solute by the
leaders of solution molarity is
abbreviated with a capital M which
stands for moles per liter and in a lot
of calculations molarity is used as a
conversion factor make sure to be
careful not to confuse molarity capital
m with moles often times students
abbreviate moles with a lowercase M and
this can be confusing when incorporating
the concept of molarity
which is abbreviated with a capital M
but remember moles and molarity are not
the same thing let's try a problem
what is the molarity of a solution
formed from six point seven five grams
of sodium chloride dissolved in water to
make a solution with a total volume of
452 milliliters remember we know that
molarity is equal to moles per liter and
so we have some amount of a solute or
moles on top and some volume of solution
or liters on the bottom in this example
problem we have neither moles nor
milliliters but we know that the amount
of solute given to us in grams must go
on the top and the volume of the
solution given to us and milliliters
must go on the bottom now we will use
dimensional analysis to convert grams to
moles and milliliters to liters to come
up with our final answer in molarity we
can use our periodic table to find the
molar mass of sodium chloride and you'll
come up with 58.4 grams of sodium
chloride per mole of sodium chloride
this allows us to cancel out grams we
now have units of moles per milliliter
but remember molarity is mole's per
liter so we need to convert milliliters
to liters we know that 1,000 milliliters
are in 1 liter by writing 1,000
milliliters on the top this allows us to
can
about milliliters and leaves us with
units of moles on the top and liters on
the bottom which is exactly what we're
looking for when we're trying to
calculate molarity when we solve this
calculation we'll find that the molarity
of the solution is 0.25 6 moles per
liter or 0.25 6 molar let's try a
problem using molarity as a conversion
factor how many grams of potassium
hydroxide are required to make 250
milliliters of a 6.0 molar solution of
potassium hydroxide often students are
confused about whether to start with 250
milliliters to begin our calculation or
the value of 6.0 molar to begin our
calculation the question is asking us
how many grams grams is a singular unit
so we'll start with 250 milliliters
because milliliters is also a singular
unit now molarity although we've
abbreviated it with a capital M means
moles per liter and and so we'll use
this value as a conversion factor to
convert to grams from milliliters or to
convert between mass and volume let's
start our problem by writing 250
milliliters now the conversion factor
that were given 6.0 molar is moles per
liter so if we want to use this
conversion factor we first need to
convert 250 milliliters to liters when
we cancel out milliliters we'll see that
we have units of liters and this allows
us to use our molarity values 6.0 molar
or 6.0 moles per liter to convert from
liters to moles note that I wrote 6.0
moles on the top of potassium hydroxide
per liter of potassium hydroxide this is
because I already had leaders on the top
and I want leaders to cancel out if we
were to stop this problem now we would
have an answer in moles of potassium
hydroxide but our question is asking us
for how many grams of potassium
hydroxide and so we need to convert from
moles to grams using the molar mass of
potassium hydroxide which is 56.1 grams
per mole
this allows us to cancel out the units
of moles and gives us a final answer of
84 grams of potassium hydroxide thanks
for watching the chemistry solution
you
