Calculating Molarity : Solutions - chemistry, Class 12 Video Lecture

all right so our next topic is relating
once again moles to lab when we take a
solution in lab we need to not just know
what it is but we need to know what
concentration is it how much is
dissolved in that solution that we're
going to be mixing and the concentration
that we use primarily is called molarity
so you can see inside of that word we
have moles again so we're talking moles
dealing with a certain volume of
solution so molarity is an expression of
concentration with units per of a units
of moles per liter but more specifically
it is moles of the solute so the grams
of a solid normally that is dissolved in
liters of solution and that's a
distinction that needs to be made it's
not liters of the solvent which does is
dissolving but liters of the solution
meaning the solvent and the solute are
mixed to equal that volume
all right so moles of solutes liters of
solution so just like before when we had
a three variable equation I showed you
how to use these triangles if you are
looking for let's say the molarity you
cover the molarity and molarity would
then be equal to moles over liters if
you're looking for moles so that would
be moles the equals molarity times
leaders and if you're looking for the
volume in liters liters would equal
moles divided by molarity okay so
molarity is abbreviated with a unit of
capital M if you have a six point zero
capital M molar solution that's how you
say that
a six point zero molar solution you
sometimes see it written like that but
we're going to use capital m capital M
the units for molarity and again you say
a molar solution a certain molar
solution 6.0 molar all right these right
here are called volumetric flasks and
this is how chemists make a solution
this is how we're going to make a
solution in lab you're going to get a
particular volumetric flow
ask volumetric flasks and you can see I
don't know how well the picture comes
out there but there's only one
graduation on these flasks and that one
graduation is equal to the volume of the
flask and so the way you make a solution
of a certain known molarity is you know
how many grams are there so you know how
many moles and then you add those grams
to the flask start adding water usually
distilled water and then mix it around
before you actually add the water up to
the line you do this is what I mean by
it is not the solvent you don't add a
thousand milliliters to the line and
then add the salad because then it's
going to push the volume greater than a
thousand because it takes up space all
right so let's try one so find the
molarity of a solution that contains
forty nine point three grams of NaCl
enough water to make so there's the key
phrase in enough water to make 500
milliliters of solution not in 500
millions of water but in enough water to
make 500 milliliters of solution so
forty nine point three grams in order to
do molarity molarity equals moles of the
solute divided by liters of the solution
so I need to change forty nine point
three grams to moles a skill we have
here to go from grams to moles
I need the molar mass the molar mass of
sodium chloride one na plus one CL is
fifty eight point five grams to mole and
so that will give me the number of moles
so forty nine point three divided by
fifty eight point five and I got point
eight four now significant digits I had
four in my volume I have three in my
mass so point eight four three moles of
NaCl so I have figured out how many
moles I have in that many grams so now
to do the concentration of the solution
I'm going to divide the moles by the
liters of solution
500 milliliters so 500 milliliters there
are 1000 milliliters and 1 liter but the
really easy ways your most of the time
going to have milliliters the reason is
these volumetric flasks are measured in
milliliters and the easy way to go from
milliliters to liters is you just go one
two three to the right so 0.5 zero zero
zero liters so that's going to carry
down to here so 0.5 zero zero zero
liters of solution moles over liters is
going to give us molarity 0.4 3 divided
by 0.5 and we got 1 point 6 8 5 so 1
point 6 9 capital M molarity or again 1
point 6 9 molar solution a one point 6 9
molar solution this calculation is
basically the same thing I just want to
point out that centimeters cubed of the
same as milliliters so once again we go
one two three to the right that would be
one point five zero zero zero liters so
the same thing we have grams we're going
to change those to moles we change the
centimeters cubed to liters divided the
two and you get the molarity so same
question a different way to work the
question how many grams are needed to me
so this is a calculation that a chemist
does that a chemistry teacher does if
you are trying to make a solution for
lab we I need to know how many
milliliters they need to make so let's
say I need 50 milliliters 60.0
milliliters and I know the concentration
needs to be 0.25 molar all right so now
looking back at our triangle of our
formula for molarity moles molarity
liters grams is going to fit with moles
because grams is something that we could
measure on our balance we can't set our
balance to moles and measure moles of
nacl but I can measure grams of NaCl so
I'm going to solve this formula for
moles and then use my molar mass to
change the grams so moles which I can
then change to grams equals so again you
hold your hand over the moles it's equal
to molarity times liters equals the
molarity 0.25 0 molar
time's the leaders I have milliliters so
one two three to the right is going to
be point zero five zero zero liters
multiply those two together 0.25 times
point zero five equals point zero one
two five moles of nacl and then one mole
is the molar mass of NaCl which we did
in the previous problem fifty eight
point five so times fifty eight point
five and I get point seven three one
grams of NaCl so how many grams are
needed to make fifty milliliters
well it takes point seven three one
grams of NaCl so we would measure out
point seven three one grams on our
balance we would put that into a fifty
milliliter volumetric size the smallest
one listed here is one hundred but they
keep going down we have 25 tens I even
have a few that are even smaller than
that so how many grams are needed to
make once again same question this time
it's already in liters so moles equals
molarity times leaders molarity times
liters that's and then one mole is 58
point five grams again now the next kind
of question that is how would you
prepare a hundred milliliters of 0.25
molar kno3 solution so we have a
different substance here so we need a
molar mass 39 point one four K 14 16
times 3 is 48 112 five six so 101.1
grams per mole so the molar mass of kno3
101.1 grams per mole how would you
prepare it so if you were asked to make
the solution what would you do all right
well we need to know how many grams I
need to measure on my balance in order
to make that volume of that
concentration so just like before our
triangle moles molarity liters I'm going
to find moles so that I can calculate
grams
so moles equals malaria
times liters equals 0.25 molar times 1 2
3 to the left point 1 0-0 liters number
the decimal point written in there means
that that is three significant digits
this one is only two significant digits
so my answer when I multiply these are
only going to be two significant digits
0.25 times 0.1 gives me 0.25 moles of
kno3
and then one mole is 101.1 grams the
molar mass so a point zero two five
times 101 point one and I get to point
now two significant digits so two point
five we would carry that further in our
balance because our balances go to two
decimal places but for this particular
problem that's how many grams of candle
three honey now that does not answer the
question though the question says how
would you prepare it it doesn't say how
many grams here's a sentence you can use
to solve all of these I would say I'm
going to dissolve however many grams I
have in this case 2.5 grams of whatever
the substance is and here's the key
phrase in enough water to make and then
whatever the volume is of the
so I'm going to dissolve 2.5 grams of
candle 3 in enough water to make 100
milliliters of solution
so that's molarity there are a few more
practice problems here I can solve for
you we have piano 3 again so again the
same molar mass 101 point 1 so what is
the molarity if it contains ninety seven
point three grams of kno3 in enough
water to make 250 point zero centimeters
cubed of solution so molarity equals
moldavite by leaders ninety three point
seven grams of candle three we figured
out the molar mass on the last problem
101.1 93.7 / 101 point one and you get
point nine to seven moles so point nine
to seven moles divided by seven s cube
same as milliliter so one two three -
left point two five zero zero liters
point nine two seven divided by 0.25 and
we got three point seven one molar
solution of kno3
okay the second one we have how many
grams are needed to make so now we're
talking grams again remember grams in
our formula are really moles because
then we can use the molar mass so moles
equals molarity times leaders equals 0.5
0 0 molar times already and liters how
about that
so 0.5 times 0.75 equals 0.375 moles of
kno3 three digits one ball 101.1 grams
so multiply that times 101 point one
gram then we got thirty seven point nine
grams of panel three if I was asking how
would I make that solution you would say
I would dissolve thirty seven point nine
grams of ko3 and enough water to make
0.75 zero liters of solution