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This content explains the fundamental principles of quantitative chemical analysis, specifically focusing on titrations as a method to determine the unknown concentration of a substance by reacting it with a solution of known concentration.
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hi everyone in this video we're going to
be talking about quantitative chemical
analysis we're going to describe the
fundamental aspects of titrations and
we're also going to learn how to perform
stoichiometric calculations using
titration data
quantitative chemical analysis is the
analysis that allows you to determine
the amount or concentration of a
substance in a sample we're going to
talk about two different types of
quantitative chemical analysis the first
is titration and that's what we'll cover
in this video and then the second is
gravimetric analysis
in titration it's an analysis based upon
the stoichiometric reaction of two
reagents or two reactants and often we
do this when we're analyzing acid-base
reactions with gravimetric analysis the
analysis based upon the mass of
something so often you'll have a
substance that you react and then you
look at the masses of the products in
order to learn information about the
original substance titrations involve
two solutions the first solution is the titrant
titrant
the titrant is a solution containing a
known concentration of reactants our
other solution is What's called the
analyte this is the solution we're
trying to analyze hence the name and
this has an unknown amount or concentration
concentration
to do a titration you'll use a piece of
glassware called a buret you can see the
puret and this photo it's this long
piece of glassware
that the student is filling so he's
filling it with titrant so that would be
the solution of a known concentration
so here this blue solution inside the
buret is what we would call the titrant
and you can see below the burat all the
way down here in this Erlenmeyer we
would have another solution and this is
the analyte the solution that we're
trying to analyze
so if you look closely at the buret and
in this zoomed in picture on the right
you would see that the buret has
markings or graduations like a graduated
cylinder this allows us to know exactly
how much titrant we start with and how
much we would add to the analyte in the
Erlenmeyer below it at the very bottom
of the buret there is a stopcock which
allows you to open and close a valve to
allow the titrant to drip into the
analyte so we drip enough of the titrant
in order to have a complete reaction
with the anilate at which point we would
achieve what we call the equivalence
point right the equivalence point is the
volume of titrant added for a complete
reaction with the analyte knowing this
volume will allow us to calculate the
concentration of the analyte how do we
know when we've had a complete reaction
of titrant to analyte we would add
what's called an indicator an indicator
is added to the analyte solution and
causes a change in color when we're near
the equivalence point of our reaction or
of our titration the volume of titrant
actually measured for expected
stoichiometric equivalence is what we
call the endpoint so the endpoint is
what you would measure when you see your
indicator change color now if you've
made a good choice for your indicator
then the equivalence point and the end
point should Vape should be very similar
the difference between them should be negligible
negligible
one example of an indicator is shown
here at the bottom right for what's
called phenolphthalein phenolphthalein
is an indicator typically used in
acid-base titrations and it has a color
change that allows us to know when we've
neutralized our acid and base so you can
see at the photo at the left
and this is when it is in acidic conditions
and then as you add more and more bass
right to make our solution basic the
phenolphthalein changes to a bright pink color
so as we see in the pictures in the
middle as we're adding more and more of
the base we start to see that pink color
in the Erlenmeyer flask the moment at
which the pink color persists is the
moment when we have effectively
neutralized all of our acid with with
base and this is what we would call the
end point based upon the use of this
indicator let's do some examples of
calculations that we would do for
titration data in this example it says
the end point in a titration of 50
milliliter sample of aqueous
hydrochloric acid was reached by
addition of 35.23 mils of 0.250 molar
sodium hydroxide titrant so it sounds
like the sodium hydroxide was inside the
buret right we drip down enough of it to
reach 35.23 mils added to our analyte
which is the hydrochloric acid
we saw some color change with our
indicator that told us that we had
reached the end point
the titration reaction is given to us
right we know how to write acid-base
neutralization reactions so using this
what is the molarity of the hydrochloric acid
acid
we have a diagram to show what our
approach should be what we're going to
do is start with the volume of sodium hydroxide
hydroxide
using the volume of sodium hydroxide and
its molar concentration which is given
right because it's known we can
calculate the moles of sodium hydroxide
once we know the moles of sodium
hydroxide we can relate this to the
moles of hydrochloric acid in our
analyte using the stoichiometric factor
from our chemical equation
once we've calculated moles of
hydrochloric acid we can use the volume
of our analyte which is 50 milliliters
to calculate the concentration of the
hydrochloric acid so this is going to be
our approach to solve this problem okay
so we said that our approach should be
first to calculate the moles of sodium
hydroxide that were added of our titrant
so we have a volume of
if we multiply this by our molarity then
we'll be able to calculate the moles of
sodium hydroxide but you can see that
our molarity will be in moles per liter
so my first step is going to be to
convert milliliters to liters so I use
my conversion factor which has one liter
in the numerator and a thousand
milliliters in the denominator
I have chosen it this way so that the
milliliters will cancel and now I'll
have a volume in liters
let's multiply this volume by the
concentration of the sodium hydroxide
titrant so this is given as 0.250 moles
of sodium hydroxide and AOH
per liter
and now we see that liters cancel and
this calculation will give us moles of
sodium hydroxide
plugging this into my calculator I
our next step should then be to use the
stoichiometric factor to determine how
many moles of hydrochloric acid we must
have reacted
so I start with the number of moles of
NaOH that I have
and I'm going to use my stoichiometric
factor to convert this to moles of
hydrochloric acid to do this I look at
this balanced stoichiometric reaction
where I would use one mole of NaOH in my denominator
denominator
and one mole of hydrochloric acid in my
numerator and I got those numbers from
the coefficients in my balanced chemical
equation now moles of NaOH will cancel
and I'll have moles of hydrochloric acid
since the stoichiometric factor is one
to one
I still have
0.00881 moles
and this gives me the number of moles of
hydrochloric acid that have reacted in
my analyte so now that I know the number
of moles of hydrochloric acid I'm ready
to calculate the concentration of the
hydrochloric acid so the concentration
or the molarity of the hydrochloric acid
is equal to the number of moles divided
by the volume we've calculated the
and our volume is given in the problem
the analytes volume was 50 milliliters
is my volume however our molarity should
be in moles per liter so I'll need to
convert milliliters to liters so again
I'll use my conversion factor I know
that a thousand milliliters
equals one liter and I've put the
milliliters in the denominator so that
they cancel appropriately
plugging this into my calculator I
calculate that the concentration of the
hydrochloric acid should be
0.176 molar
or moles per liter
let's try one last example in this
example our question is what volume in
milliliters of .0380 molar barium
hydroxide is required to neutralize 70
milliliters of 0.05 molar hydrochloric acid
acid
now in this problem we're not given our
balanced chemical equation and since
this problem requires knowing the
Stoichiometry this is where we should
start so let's write our balanced
now we learned earlier that when we have
the neutralization reaction for an acid
in a base that it is going to produce a
salt and water our salt in this case
will be comprised of the barium from the
barium hydroxide and the chloride from
the hydrochloric acid
now in barium and chloride react to form
our ionic compound the formula should be
bacl2 because barium has a plus two charge
charge
and then we said we would also form water
water
as the product of our neutralization
reaction so now I need to make sure that
this reaction is balanced because of the
cl2 in my barium chloride I'm going to
need to put a 2 in front of the
hydrochloric acid
looking at my reaction the other thing
that needs to be changed in order to
make this balanced is that I need a 2 in
front of the water right because I have
two hydroxide ions here that would
contribute to give me two oxygens for
example so I need to have the two in
front of the water
now that our reaction is balanced let's
look again on our question to make sure
we understand what we're trying to find
the question says what volume in
milliliters of barium hydroxide is
required to neutralize a given volume
and concentration of hydrochloric acid
so what we're trying to find is the
volume of the barium hydroxide we are
given more information about the
hydrochloric acid and so that is where
we're going to start
of HCL
if we multiply the volume of
hydrochloric acid by its molarity that
will that will calculate moles of
hydrochloric acid
because this volume is given to us in
milliliters again we're going to need to
use our conversion factor to convert
from milliliters to liters
now that our volume is in liters we can
multiply by our concentration of the
hydrochloric acid
in order to calculate moles of
hydrochloric acid
the leaders cancel
and plugging these numbers into my
calculator I calculate that I have neutralized
neutralized
.00350 moles
of hydrochloric acid HCL
all right so now that we know the moles
of hydrochloric acid we can use the
Stoichiometry and our chemical equation
to determine how many moles of barium
hydroxide must have reacted so I start
this gives me the moles of hydrochloric acid
acid
and I'm going to use my stoichiometric
factor to then calculate the moles of
barium hydroxide I'm going to need the
moles of hydrochloric acid to be in the
denominator so that these cancel my
Stoichiometry tells me that this should
be two moles of hydrochloric acid for
so this will give me moles of barium
hydroxide they want us to find the
volume of the barium hydroxide solution
so if I have moles of barium hydroxide I
can use my concentration to calculate
what that volume should be
the concentration is .0380
.0380
for every one liter
so if I use my concentration in this way
moles of barium hydroxide will cancel
since my question is asking for a volume
in milliliters my last step is to
convert from liters to milliliters so
again I'm going to use my conversion factor
factor
and this time leaders needs to be in the
all right doing this last calculation I
find that the volume that is needed is
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