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For the compound MgO, what percentage of its mass
is made up of Mg atoms, and what percentage of
made up of O atoms?
Look up the
atomic masses
Mg = 24.312 O = 16.000
on the periodic
table for each
Although there is a 1:1 ratio in terms of the number of
atoms, the masses differ and so do the percentages.
Therefore Relative Formula Mass (Mr) is 24.312 + 16.000
= 40.312
We can now work out the percentages:
(24.312 / 40.312) x 100 = 60.3% of mass due to Mg
(16.000 / 40.312) x 100 = 39.7% of mass due to O…read more

Slide 3

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The Mole
· If we weigh the Carbon atom, it doesn't weigh very much!
· But if we add enough Carbon atoms so that it weighed exactly the same as
its Relative Atomic Mass (RAM), (which is 12g mol-1) then 6.022 x 10^23
atoms would be needed.
· This number is Avagadro's Constant, and it is called the Mole.
There are
6.022 x 10^23
atoms of
Carbon on the
0.00g exactly 12 g.
12.00g…read more

Slide 4

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1 mol of a substance weighs its Mr
What is the mass of 1 mol of CuO?
Mr (CuO) = 63.55 + 16.00 = 79.55
Therefore 1 mol of CuO weighs 79.55g.
We can think of
Mass : Mole this as a ratio.
Mr: 1
What is the mass of 0.23 mol of CuO?
Mass : Mole
Mr : 1
We can think of
63.55g : 1 CuO
the ratio as 14.62
g : 0.23 mol 63.55x0.23: 0.23
= 14.62g…read more

Slide 5

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You can also do it the other way...
I have 10.6g of Cu. How many moles of copper is this?
Mass : Mole
Mr : 1
63.55 : 1 Cu
1g : 1/63.55
10.6g : x ans by 10.6
= 0.167…read more

Slide 6

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Mole Triangle
Putting the mass calculations into a simplified form, such as
this triangle, allows to easily convert between moles and mass.
Mass = Moles x Mr
Moles = Mass / Mr
Mr = Mass / Moles
Moles Mr…read more

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