A Level Chemistry – The Mole Concept

hey guys welcome back the next series of

videos are all about looking at some

fundamental ways in which we quantify

amounts of different substances in

chemistry this branch of chemistry is

called quantitative chemistry but no

need for fancy words here it literally

is just looking at and calculating

amounts of stuff

now there's literally no way we can look

at the amount of something in chemistry

and not come across the infamous mole

it's actually really great that you're

watching this video because the mole is

probably the most important thing you'll

learn for a little while okay so to get

in the spirit a bit I want you to

imagine having two separate collections

of basketballs and golf balls weighing

ten kilos each now think about the

difference in the amount of balls

between the two you don't need to be a

dreamiest to know that they'll

definitely be more golf balls than

basketballs 218 compared to 16 in fact

and this is the case even though the

total masses are equal to each other you

can play a lot more games of golf than

you can basketball I see you in you

don't lose heaps of balls in the water

like I did when I tried so why did I

bring up this example well because this

exact same idea exists in chemistry when

we deal with particles of different

sizes we're easy able to determine mass

by weighing something but measuring an

amount of substance by mass doesn't

directly give us the amount of particles

in that substance we need to know

amounts of particles to predict

reactions and this is where moles come

in to link the mass of a substance with

the number of particles in it you can

check out the attached syllabus dot

points on the video page to see where

all of these content comes from so we'll

firstly look at what moles actually are

and from this we'll do a couple of

conversions between an actual number of

particles and a number of moles of that

particle we have will then put on our

thinking caps and actually try to make

sense of the theory surrounding them all

concept and how this ties in to the

molar masses of different substances and

finally we'll link this all really

briefly into the idea of stoichiometry

which sounds really complex but trust me

it isn't so with all that to do

let's get cracking with an introduction

to the concept of moles okay whenever we

measure things we use units to help us

organize our quantities for example if

we wanted to measure the length of this

video we'd probably use seconds or

minutes as our units of time as these

are pretty widely known and convenient

but if I were to tell you that the time

of recording this video I was born

eleven million nine hundred and eighty

seven thousand six hundred and forty

minutes ago you'd think it was a right

weirdo and that's because when you're

asking the age of somebody you typically

expect the answer to be in years

now I'll leave it to you to figure out

how old I am in years but chemists are

always dealing in numbers of atoms and

molecules and if you think about it this

could get pretty tricky as these are so

tiny and there are so many substances we

want to measure even a small amount of

matter contains trillions of atoms so it

makes sense that we would use a large

number to help us count the number of

atoms or molecules in a substance that

specific number 6.022 times 10 to power

23 is known as Avogadro's constant it

gives us the number of particles in a

mole don't worry about needing to

remember this it's going to be given to

you on your formula sheet so in other

words no matter what substance you're

talking about we say a mole of it is

6.022 times 10 to the 23 particles of it

so now that we know what a mole is it

makes sense that we should be able to

convert between an actual number of

particles a number of moles we know that

there is avocados number of particles in

one mole so converting from number of

moles which is written as a lowercase n

to particle number we simply multiply by

Avogadro's number which is given by this

symbol and it's made pretty obvious by

this little a here of course the

opposite direction applies to converting

from number of particles to number of

moles requires division by Avogadro's

constant okay great so let's do a couple

of examples there's six moles of a

substance how many particles is this

well 6 multiplied by Avogadro's constant

gives us 3 point 6 1 3 times 10 to the

power 24 all right if I have 11 times 10

to the power 23 atoms how many moles do

I have

Wow 11 times 10 to power 23 divided by

Avogadro's constant equals one point

eight three moles ah easy-peasy so I can

almost hear you guys saying yeah it

makes sense that we use a really big

number to count molecules and atoms but

6.022 times 10 to the 23 seems a bit

random down there well you're right it

is a pretty weird number but it's like

that because it's been specifically

designed by chemists to be the most

useful number possible one mole is

defined as the exact number of carbon-12

atoms weighing 12 grams carbon-12 is

chosen as the reference point for most

things to do with comparing elements and

so this is no different this is so

useful since we can use this to relate

the mass of a substance to the amount of

moles the atomic weights or relative

atomic mass on the periodic table is in

atomic mass units amu and tells us the

mass of one mole of a substance so

looking at our atomic weights we know

that hydrogen has an atomic weight of

one point zero one atomic mass units

meaning that one mole of hydrogen atoms

will weigh one point zero one grams in

other words one point zero one grams of

hydrogen has six point zero two two

times ten to the power 23 particles in

it furthermore oxygen has a relative

atomic weight of sixteen point zero zero

eight amah class units meaning one mole

of oxygen atoms will weigh 16 point zero

zero grams and so on just a little point

to notice here hydrogen and oxygen

typically exist as diatomic molecules

which is just a molecule made of two

atoms at the same element so if we were

asked to find the molar masses of

hydrogen or oxygen gas we'd need to

multiply these values by two if you guys

were at all unsure about how we get

these numbers for our relative atomic

weights that's all good just go back and

check out the past videos on relative

mass so to tell the mass of one mole of

an atom which is called the molar mass

it's literally as easy as reading the

atomic weight of the corresponding atom

off the periodic table and writing this

in terms of grams again if this is

seeming totally new to you and you want

to check it out in more detail head back

to the video called relative math part

two we are then able to use this same

idea when talking about molecules as

these are just combinations of atoms for

example one molecule of water is made up

of two hydrogen's and one oxygen meaning

its molecular weight is equal to 2 times

the atomic weight of hydrogen plus the

atomic weight of oxygen which equals 18

point o2 that means that one mole of

water molecules will weigh eighteen

point zero two grams okay I think it's

important here to clear up a little bit

of terminology that can be a bit

confusing the difference between

molecular weight and molar mass

molecular weight is simply the combined

atomic masses of all the atoms in a

molecule written as number this is

obviously going to be a tiny mass and so

it's in atomic mass units but the key

thing is that the molecular weight is

equal to the mass in grams of a mole of

that molecule so to convert this into a

molar mass M R which is the mass of one

mole of a particular molecule we simply

write in the unit's grams per mole after

it's really that simple

using our molar mass will become really

useful in our master mole calculations

which we'll dive into in another video

so definitely tune in for that so what

on earth two moles have to do with our

chemical reactions well this is where we

introduce the term stoichiometry simply

per stoichiometry is the idea of using

ratios in chemical equations to

calculate amounts of substances being

used up or produced in a reaction so

just looking at our reaction to produce

water before we thought of the numbers

here is referring to the number of atoms

involved in the reaction but of course

since moles are just a way of counting

atoms the same ratio will apply to moles

as the amount of atoms so we can now

read into this is telling us that for

every two moles of hydrogen gas

and one mole of oxygen that react to

moles of water molecules are formed

these are known as the molar ratios of

this equation we can then use the same

ratio to tell us that one mole of

hydrogen gas and half a mole of oxygen

gas react to produce one mole of water

and so on all right guys that's really

about it for moles so I'm gonna put the

important points to take away from this

video up on screen now just one other

thing to know as we progress through the

course and do even more calculations the

concept of moles will continually pop up

so it's not as though this is a one-time

learn type of thing with some practice

these concepts should become a pretty

effortless part of some more difficult

questions anyway I'll catch you guys in

the next one