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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

you