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GCSE Chemistry - The Mole (Higher Tier) #24

in this video we're gonna cover what the

term model means and see how we can use

this formula to convert between moles

mass and relative formula mass

now the term mole is a bit weird but

really it is the unit we use to measure

the amount of chemical that we have so

just like we measure distance in meters

and time in seconds we can measure how

much substance we have in moles and 1

mole of any substance is the amount of

that substance that contains a 6.02

times 10 to the 23 particles with

particles referring to atoms molecules

ions or even electrons depending on

which substance you're talking about

so if we had a little pile of carbon and

we were told that it contained exactly

one mole of carbon then there must be

6.02 times 10 to the 23 atoms of carbon

in that pile

we call this number Avogadro's constant

and the reason is this the civic number

is that the mass of that many particles

of any substance will be exactly the

same number as the substances relative

atomic or formula mass in grams

so our little pile of one mole of carbon

would weigh exactly 12 grams because

carbons relative atomic mass is 12

meanwhile one mole of oxygen which has

the relative formula mass of 16 times 2

so 32 would weigh 32 grams

or one mole of co2 with a relative

formula mass of 12 plus 16 plus 16 and

so 44 would wave 44 grams

but in all these cases there would be

6.02 times 10 to the 23 atoms or

molecules

because of this rule we can create a

formula which tells us that the number

of moles in a sample is equal to the

mass of that element or compound divided

by its Emma

so if we want to know how many moles

there were in 40 2.5 grams of ammonia we

would do 42 point 5 which is the mass

divided by 14 plus 3 times 1 so 17 which

is the AMR of ammonia

and this would give us 2.5 so we know

that there are two and a half moles of

ammonia in the 42.5 grams

we can also rearrange the formula to

find the mass if we were given the

number of moles for example what's the

mass of three moles of carbon dioxide

well this time weird multiply our three

moles by the mr of co2 which is 12 plus

2 times 16 so 44 which gives us 132

grams of co2

we can also work out the mass of a

particular element within a larger

compound like the mass of carbon in

three moles of carbon dioxide

for this all we do is take the number of

moles which is 3 and multiply it by the

EMA of the carbon which is 12 so 3 times

12 which gives us 36 grams of carbon our

132 grams of carbon dioxide

and if you wanted to go one step further

you could subtract that 36 grams from

the original 132 grams to find that

there must be 96 grams of oxygen because

we can see that co2 is only made up of

carbon and oxygen

the last thing we need to mention is

that when you look at a chemical

equation you can think of them in terms

of moles so for this equation we can

think of one mole of magnesium reacting

with two moles of hydrochloric acid to

form one mole of magnesium chloride and

one mole of hydrogen gas

and you should think of these as ratios

so if we started with two moles of

magnesium would have to react it with

four moles of hydrochloric acid and it

would produce two moles of magnesium

chloride and hydrogen gas

anyway that's all for today so I hope

you found it useful and I'll see you

next time

you