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Chapter 7.3c More Stoichiometry Problems | Michelle Bunagan | YouTubeToText
YouTube Transcript: Chapter 7.3c More Stoichiometry Problems
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This content explains how to perform stoichiometric calculations that involve converting between mass, moles, and solution molarity, using a provided balanced chemical equation and relevant conversion factors like molar mass and density.
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hi everyone in previous videos we
learned how to think about a balanced
chemical equation and these
stoichiometric factors that we can write
from that equation we've also learned
how to do simple calculations to convert
from one species to another in a
reaction and this video is going to do a
little bit more practice performing
stoichiometric calculations involving
Mass moles and solution molarity
so let's look at some examples first we
have a question asking what mass of
sodium hydroxide NaOH would be required
to produce 16 grams of the antacid milk
of magnesia so this is magnesium
hydroxide by the following reaction and
so we're given the balanced chemical
equation and we want to go from the mass
of the antacid right the milk of
magnesia to a mass of sodium hydroxide
right so how much sodium hydroxide would
we need to produce these 16 grams of the
magnesium hydroxide
now since both of these species are
involved in the chemical equation we
know that we can relate the moles of
magnesium hydroxide to the number of
moles of sodium hydroxide right using
the Stoichiometry of the equation but
this question is asking us to relate the
mass of these two species so if we start
with a certain mass of magnesium
hydroxide we would first need to figure
out how much in terms of the number of
moles we have of magnesium hydroxide
before we then can relate the moles of
magnesium hydroxide to moles of sodium
hydroxide and then finally since we're
looking for the mass of sodium hydroxide
we would need to use the molar mass of
that species to figure out the mass in
that certain number of moles so let's do
this problem all right so we said that
we have a desire to produce 6 15 grams
of the antacid milk of magnesia so this
all right and according to our flow
diagram what we first want to do is
convert from grams of magnesium
hydroxide to moles of magnesium
hydroxide and to do this I need to use
I can see from where I've started right
I have grams of magnesium hydroxide I'm
going to want to have that cancel so
I'll want grams in my denominator so it
grams of magnesium hydroxide in one mole
all right so grams of magnesium
hydroxide are going to cancel and now
we're in moles so now we can use our
now there's different ways for us to
write our stoichiometric Factor right
depending on which species we put in the
numerator in which in the denominator
here I can see that all want moles of
magnesium hydroxide to cancel so I'll
want that species to be in the denominator
and I'm trying to relate this to the
number of moles of sodium hydroxide so
and then I need to look at the
coefficients for my balanced chemical
equation I can see that the coefficient
for the magnesium hydroxide is one
whereas the coefficient for the sodium
hydroxide is 2. so that gives me the
numbers to put into my stoichiometric Factor
Factor
all right so now we've canceled moles of
magnesium hydroxide and we are now in
moles of sodium hydroxide so it looks
like we have one more step the question
is asking what mass of sodium hydroxide
would be required
so we would need to convert from moles
of sodium hydroxide to a mass and here
I'm going to need to use my molar mass
for sodium hydroxide in this case I want
moles to cancel right since I have moles
here in my numerator I'm going to put
moles of NaOH
in my denominator so one mole of NaOH is
all right so now moles of sodium
hydroxide will cancel and the number
that I calculate will have the units of
grams of sodium hydroxide
all right I'm putting these numbers into
a calculator I calculated that 22 grams
of sodium hydroxide would be required to
produce the 16 grams of magnesium hydroxide
there's a nice flow diagram in your
textbook which helps us think about
these more complex calculations
involving stoichiometric factors
so as we can see in the middle of the
flow diagram we're able to relate A and
B right if these are substances involved
in a chemical reaction and we have a
balanced chemical equation for the
reaction we should be able to relate the
number of moles of a to the number of
moles of B using this stoichiometric
Factor right based on the coefficients
of these substances in the balanced
chemical equation
but often in the laboratory we don't
know how many moles we have instead we
measure other parameters right so we
might measure the mass of substance a
and so just as we did in the last
problem we would need to use the molar
mass to convert from the mass of a to moles
moles
another parameter we might measure
instead of mass would be the volume of a
pure substance so say we have a liquid
substance that's pure we can measure its
volume with a graduated cylinder then we
would need to figure out how we could
get from the volume of the substance to
moles and what we would need to use here
would be the density so density would
help us to go from a volume to a mass
and then once we have the mass we can go
to moles using the molar mass
another parameter we might have instead
of moles would be the volume of a
solution that contains substance a so
for example maybe we have an aqueous
solution containing the solute a we know
that we have a certain volume of that
solution how would we calculate the
number of moles of a well to do that we
would use the volume and the molarity in
particular the volume times the molarity
would help us to calculate the moles of
that substance as a solute in that solution
solution
and then another parameter that you may
see in your calculations would be
instead of having moles of a we may have
the number of particles of a so we know
that we can convert from numbers of
particles to moles using Avogadro's
number and so once we use this we can
then relate the moles of a using the
stoichiometric factor to moles of B
so let's try one more example to make
use of what we learned from the flow
diagram so in this case we have the
information that Automotive airbags
inflate when a sample of sodium azide
nan3 is very rapidly decomposed and we
have our balanced chemical equation
right so we have two moles of sodium
azide decomposed to give two moles of
sodium which is solid and three moles of
nitrogen gas
so our question is what mass of sodium
azide is required to produce this is 2.6
cubic feet or 73.6 liters of nitrogen
gas with a density of 1.25 grams per liter
liter
all right so we can see that we're
trying to relate the nitrogen gas which
we see here as a product to the mass of
sodium azide the reactant that we start with
with
all right so instead of moles here we
have other parameters right we want to
find the mass of sodium azide required
and here in terms of the nitrogen gas we
want to produce 73.6 liters so a volume
so where to start in this question we
need to start with a specific piece of
information that we're given how much we
want to produce in this case
and let's reference the flow diagram
that we saw on the previous slide
so if we know a volume right that puts
us up here on the flow diagram we can
relate the volume to the mass of that
substance using the density and then
once we have the mass we can use the
molar mass to get moles of that substance
all right so our flow diagram told us to
use the density
so we're given the density here it is
1.25 grams per liter
now I'm going to want to have liters
cancel so I'll put that in the
denominator and grams in my numerator
and I have 1.25 grams for every liter so
now liters of nitrogen cancel this is
grams of nitrogen
all right and then our flow diagram said
once we have grams right we can use the
molar mass to convert to moles
so the molar mass of this substance
right this is nitrogen is going to be 28
grams per mole now I want grams to
cancel so I'll put grams in the denominator
so we have 28 grams of nitrogen for
so now grams of nitrogen cancel and I am
in moles of nitrogen so now that I'm in
moles I can use my stoichiometric factor
to convert from moles of nitrogen to
moles of sodium azide
from my calculation I can see that I
want moles of nitrogen to be in the denominator
and so then I want moles
of the sodium azide
to be in my numerator looking at my
balanced chemical equation I can see
that the coefficient for nitrogen is
three so I'll put that down here and the
coefficient for the sodium azide is two
so that'll go up in the numerator
now moles of nitrogen can cancel and I
am in moles of sodium azide let's look
at my question again it says what mass
of sodium azide is required right so now
that we're in moles of sodium azide it
looks like we need to finish the problem
by converting to Mass
so for this we can use our molar mass
the molar mass of sodium azide is 65
grams per mole it looks like I need
moles to cancel so I'll put that piece
so moles of sodium azide goes in the
denominator and the mass of that mole
and moles of sodium azide cancel
and we can plug this into our calculator
to get the number 142 my unit should be
grams and this would be of the substance
sodium azide my reactant so when 142
grams of sodium azide react they
generate 73.6 liters of nitrogen gas
that's quite a large volume so that's
going to inflate your airbag and protect
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