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in this video we're going to talk about
how to calculate the molarity of the
solution or the concentration of the
solution so first you need the equation
molarity is equal to the moles of solute
divided by the liters of solution so
it's moles divided by volume now you
need to know what is meant by the term
solute and solution so let's say if we
dissolve salt and water individually the
salt is the solute water is the solvent
the solvent dissolves the solute and
together the combination of the solute
plus the solvent is the solution so keep
that in mind so that'll help you to
distinguish two solute from the solvent
so now let's work on this example let's
focus on Part A point 25 moles of sodium
chloride in 300 milliliters of solution
what is the molarity of that solution so
we're given everything that we need we
have the moles of the solute sodium
chloride dissolves in a solution and we
have the volume of the solution however
we need to convert it to the units
leaders and to convert milliliters to
liters simply divide by thousand one
liter is equal to a thousand milliliters
so three hundred divided by a thousand
it's going three another thing that you
can do is move the decimal three units
to the left so let's divide this by 0.3
liters and point to five divided by 0.3
that's about twenty eighty three so
that's the concentration of the first
solution in Part A now let's move on to
Part B
now keep this in mind molarity is moles
of solute divided by liters of solution
this will be helpful throughout parts B
and C so for Part B I'm going to do more
of a conversion process let's start with
60 grams of sodium hydroxide now I need
to get moles I want to have unit moles
on top and on the bottom the unit liters
so let's convert grams to moles and we
need to find the molar mass of sodium
hydroxide to do that so we need to add
up the atomic mass of sodium oxygen and
hydrogen so sodium is about twenty-two
point nine nine oxygen of sixteen
hydrogen is one point zero zero eight so
this is thirty nine point nine nine
eight but for all practical purposes
let's round it to forty so we can say
that one mole of sodium hydroxide has a
mass of forty grams so now we no longer
have the unit grams of sodium hydroxide
now all I need to do is take the moles
and divide by the leaders but we need to
convert 250 milliliters to liters so if
you take 250 and divided by thousand
you'll get the volume in liters which is
point 25 liters so notice that we have
unit moles on top and liters on the
bottom whenever you have that your
answer is the concentration in molarity
so it's 60/40 and then take that result
divided by 0.25 so the concentration is
6 molar or 6 moles per liter so that's
the answer for Part B
Part C let's find the molarity of the
solution when 700 milligrams of
potassium iodide is dissolved in 200
milliliters of solution so let's start
with what we're given now we need to
convert milligrams to grams and then
grams to moles and then take the number
of moles divided by the leaders one gram
is equal to a thousand milligrams and
now we need the molar mass of ki the
atomic mass of potassium is thirty nine
point one and the atomic mass of iodine
is 126 point nine so the sum is 166
grams per mole so one mole of ki has a
mass of 166 grams now the last thing we
need to do is take the moles divided by
the number of liters in the solution so
200 milliliters if you divide that by a
thousand that's point two liters and so
that's going to give us the answer so
it's seven hundred divided by thousand
divided by 166 divided by point two so
the concentration is point zero two one
one
so that's the molarity of the potassium
iodide solution number two fifty
milliliters of ethanol is dissolved by
four hundred milliliters of water what
is the concentration of ethanol in the
solution so which one is the solute and
which one is the solvent the solute is
dissolved by the solvent the solvent is
water in this problem the solute is
ethanol typically the solvent is usually
a
greater quantity than the song we have a
lot more water than ethanol so water
dissolves Evan on this case now in order
to find the concentration we need to
calculate the moles of the solute so we
got to get the moles of ethanol and then
divided by the leaders of the solution
now if we call the solute plus the
solvent combined makes up the solution
so what is the volume of the solution we
have 50 milliliters of solute 400
milliliters of solve n so therefore we
have 450 milliliters of solution and if
you divide that by thousand that's point
four five liters of solution so we're
going to use that later so now we need
to focus on finding the moles of the
solute which in this case is ethanol
so we have the volume and we have the
density combined we can use that to get
the grams of ethanol and then convert
that into moles by the way here is the
formula for ethanol c2h5oh so you can
always look that up on the internet or
something so let's start with the volume
of ethanol we have 50 milliliters of
ethanol and the density of ethanol is
point seven nine grams per milliliter so
now we have the mass of ethanol in grams
so now we can convert that to moles but
we got to find the molar mass so s ml
contains two carbon atoms six hydrogen
atoms and one oxygen atom in a single
molecule so this is two times twelve
point zero one plus six times one point
zero zero eight plus sixteen
so this adds up to forty six point zero
six eight grams per mole so that's the
molar mass of ethanol so one mole is
equal to forty six point zero six eight
grams now the last thing we need to do
once we have moles is to divide by the
number of liters which we know it's 0.45
so once you have these two units two
moles divided by liters you now have the
molarity of the solution so it's fifty
times point seven nine divided by forty
six point zero six eight and then take
that result divided by 0.45 so the
concentration is one point nine zero
five so that's the molarity of the
solution number three the concentration
of aluminum sulfate is point three moles
per liter what is the concentration of
the aluminum and sulfate ions well first
let's write the chemical formula of
aluminum sulfate aluminum is found in
group three a of the drag table so it
has a positive three charge and sulfate
is one of those probably sonic ions that
you should know by now
it's so4 to - and so the chemical
formula is going to be al2 so4 3 and the
concentration is point 30 moles per
liter so based on that what is the
concentration of the aluminum plus three
cation so notice that we have a
subscript of 2 so it turns out that to
find it all you have to do is multiply
the molarity by two so it's going to be
point sixty and in the case of sophi
there's three of them per formula unit
so it's going to be point thirty times
three which is point ninety
now for those of you who will need to
show your work here's what you can do
the first thing you should do is write
the dissolution of aluminium sulfate
aluminium sulfate breaks down into two
aluminium cations and three sulfate ions
so you could say this is the solid phase
and on the right the aqueous phase so
I'm going to represent this as point
three moles of aluminium sulfate
per liter because that's what it is
molarity is mole's per liter now we can
use the molar ratio to convert from I'm
going to use sulfate as an example so
from aluminum sulfate to sulfate so 1 2
3 ratio so for every 1 mole of aluminum
sulfate that dissolves 3 moles of
sulfate ions are released into the
solution so the unit moles of aluminum
sulfate will cancel so now what you have
left over is moles of sulfate per liter
of solution so it's point 3 times 3 and
so this will give you point 9 moles per
liter which you could say that's point 9
molar sophie so that's the concentration
of sulfate that's how you could show you
work if you want to but the simplest way
to get the answer is to take point 3 and
multiply by the subscript and that will
give you point 9 number 4 15 grams of
iron 3 chloride is dissolved in 450
milliliters of solution what is the
concentration of chloride so let's get
the answer just completely using
conversions and dimensional analysis
let's start with 15 grams of fecl3 now
in order to get the concentration we
need to convert this into moles and at
the same time we need to change the
substance from fecl3 to cl- so that's
going to require an extra step but first
let's calculate the molar mass of fecl3
so we have 1 iron atom and 3 chlorine
atoms the atomic mass of Fe is 55 point
eight five and the atomic mass of CL is
35 point four five
so you should get a molar mass of 160
2.2 grams per mole
so one mole of se CL 3 has a mass of 160
2.2 grams so now let's change the
substance from fecl3 to CL - there's one
chloride ion perform a unit of fecl3 so
therefore we can say that 1 mole of
fecl3 will release 3 moles of chloride
ions in a solution it's a 1 to 3 ratio
now that we have the moles of chloride
ions all we need to do is divided by the
liters of solution and that will give us
the concentration of chloride so let's
divide this 5000 to convert it to liters
so that's 4 5 liters so this is going to
be 15 divided by 160 2.2 times 3 divided
by 0.45 so the concentration of chloride
in a solution is point six one seven
moles per liter or capital n so that's
the molarity of the solution of the
chloride in a solution number five what
mass of ammonium chloride is required to
produce 250 milliliters of 0.75 molar
ammonium chloride solution so we're
given the molarity and the volume how
can we find a mass the best way is to
convert it I'm going to start with the
molarity of the solution so keep in mind
capital M molarity you can represent it
as moles per liter so a point seven five
molar solution means that we have point
seven five moles
of ammonium chloride per liter of
solution now we're not going to change
the substance so we don't need to write
nh4cl so I'm just going to write this
point seven five moles per liter now we
have the volume of the solution 250
milliliters and if we divide that by a
thousand that's point two five liters so
you want to multiply the molarity by the
volume because if you notice the unit
leaders will cancel and now we have the
moles of ammonium chloride so all we got
to do is change moles to grams and we
can do that using the molar mass so in
nh4cl we have one nitrogen atom four
hydrogen atoms and the CL atom nitrogen
is 14 point zero one H is one point zero
zero eight and CL is 35 point four five
so the molar mass is fifty three point
four nine two now because we have the
unit moles on the top left we need to
put the moles on the bottom so one mole
of nh4cl has a mass of forty three point
I mean fifty three point four nine two
grams so this is going to be 0.75 times
point two five times fifty three point
four nine two so the answer that I got
is ten point zero three grams of nh4cl
so if you're given the volume and the
molarity and you want to find the mass
you need to take the molarity and
multiply it by the volume in liters and
then multiply by the molar mass of the
substance that's going to give you the
grams of the substance number six what a
volume is required to dissolve eight
point five grams of potassium nitrate to
produce a point 15 molar potassium
nitrate solution so this time we're
given the mass of the solute the
molarity of the solution and our goal is
to find the volume we're going to find
the volume first in liters and then
we're going to convert it to milliliters
so how shall we begin now if we want to
get liters on top that means molarity
has to be on the bottom we don't want to
start with malaria because that's going
to be moles per liter however we need to
flip it at some point because we want
this unit to be on top just keep that in
mind so let's start with the mass eight
point five grams of kno3 and here's the
basic setup that I'm going to do I'm
going to start with the grams and then
using the molar mass of the substance
this will give me the moles and using
the molarity I'm going to get the
leaders on top which is what I want so
that's just a blueprint of how I'm going
to set up the conversion process and I
don't like when the whole thing just
disappears like that so we need to find
the molar mass of kno3
the tascam has an atomic mass of 39.1 n
is 14 point zero one and O is 16 3 times
16 that's 48 and 48 plus 14 point O 1 at
62 point O 1 and then let's add the 9.1
to that so this is 101 point 11 so
there's 101 point 11 grams per 1 mole of
substance now let's use the molarity of
the solution so a point 15 molar
solution means that there's point 15
moles of solute per one liter of
solution in order to get the unit moles
to cancel we need to flip this fraction
so we're going to put the point 15 moles
on the bottom and 1 liter on top so it's
8 point 5 divided by 101 point 11 and
then take that result divided by point
15 so it's point 5 6 liters so that's
the volume in liters and if you want to
convert that to milliliters all you need
to do is to multiply by thousand there's
a thousand milliliters per liter so this
is going to be about 560 milliliters
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