Masses of Atoms
(Part 1)
Relative Atomic
Mass (Ar) tells us
the mass of an
atom compared to
The Ar takes into
account all the
isotopes of the
element, so that is
why Chlorine is
We can use relative
atomic masses to work
out the relative formula
mass (Mr) of a molecule
or compound.
If we add up all the RAM nummbers present
in a molecule or compound we get the
Relative Formula Mass
2 X Hydrogen + 1 X Oxygen
H2O =
= 2X1
+ 1 X 16
= A formula mass of 18
Ar Numbers
Split into 2 halves
1) (NH4)3
= 14 X 1
NH4 = 18
(NH4)3 = 18 X 3 = 54
2) PO4
P = 31 X 1
O = 16 X 4
PO4 = 95
54 + 95 = a formula mass of 149
Calculate the relative formula mass of the
following compounds (showing your working!):
(a) Carbon monoxide CO
(c) Sulphur dioxide SO2
(d) Calcium carbonate CaCO3
(e) Sodium hydroxide NaOH
(f) Sulphuric acid H2SO4
(g) Copper sulphate CuSO4
Extension Activity
(h) Lead nitrate Pb(NO3)2
(i) Calcium hydroxide Ca(OH)2
Part 2: The Mole
Units of Measurement:
We use units of measurement in life to give us an
easier way to see large numbers.
metre = 100 cm, litre = 1000 ml, kilogram = 1000 g
The mole is a unit of measurement
In chemistry we use it to tell us how many
atoms or molecules there are
The Mole
If I had 1 mole of grapefruits I would have
602,214,150,000,000,000,000,000 (6.02 X 1023) grapefruits
which would be the same size as
If I had 1 mole of H2O I would have
6.02 X 1023 molecules of water.
How big is a mole?
• A mole of sand grains would cover the United States in
approximately one centimetre of sand.
• A human body contains roughly one hundred trillion cells;
there are roughly six billion people on Earth; so the total
number of human cells on the planet is very close to one
• If you had exactly one mole of sheets of paper, you could
make one million equal stacks from sea level on the earth
that would pass the sun.
• If you had a mole of pennies, you could give out enough
money to everyone in the world so that they could spend a
million dollars every hour, day and night, for the rest of
their lives.
The Mole
If I could weigh 6.02 X 1023 molecules of water
it would weigh 18g
Q) What is the formula mass of water?
Q) What is the relationship between the
formula mass and 1 mole of a chemical?
1 mole of a chemical is equal to
the Formula Mass in grams
I have 40g of NaOH
The Formula Mass (FM) = 40
Therefore I have 1 mole NaOH
1 mole
2 mole
0.5 moles
0.25 moles
= 40g
= 80g
= 20g
= 10g
• What is the relative formula mass of
Magnesium chloride?
• What is the relative atomic mass of Na?
• What is the mass of 0.2 moles of sulphur?
• What is the mass of 0.25 moles of CaCO3
• How many moles in 14 g of silver?
• How many moles in 0.56 g of CaO?
Triangle to help you:
Atomic mass
or Formula
Part 3
Percentages of elements in
• Write down the Ar of every element in the
• Divide each one by the total Ar.
• Times each one by a hundred.
• E.g. Magnesium oxide
– Mg =
– Oxygen =
Part 4: Empirical Formula
• The empirical formula is the simplest ratio of
the different atoms in it. For example for
ethane it is CH3.
• You can work out the empirical formula of a
compound if you know the mass of each
element in it.
Example: What is the empirical formula of a
compound that contains 27.3% carbon and
72.7% oxygen by mass?
• Step 1 Find, and write down the relative atomic masses of
the elements involved.
• Ar (C) = 12 and Ar (O) = 16
• Step 2 Turn the percentages into grams.
• There is 27.3g of carbon and 72.7g of oxygen
• Step 3 Work out how many moles of each element
this must be using
number of moles =mass
• number of moles of carbon= 27.3 =2.28
• number of moles of oxygen = 72.7=4.55
• Step 4 Divide both numbers of moles by the smallest
number (2.275 in this case).
• number of moles of carbon = 2.28 = 1
• = number of moles of oxygen = 4.55 =2
In the empirical formula, there are 1 carbon atom
and 2 oxygen atoms, so this is ……..
Example, A compound contains 75g carbon and
25g hydrogen. What is its empirical formula?
Convert to moles ( /Ar)
/12 = 6.25
/1 = 25
Divide by the smallest
6.25/ 6.25
Empirical formula
1 (a) Attempt: An oxide of carbon contains 27% carbon and
73% oxygen. What is its empirical formula? (this was the first
example, so don’t do again if you have taken good notes!)
1. (b) Confirm: Fluorspar is made of calcium and fluorine. If
51% is calcium, calculate the empirical formula.
2. (a) Attempt: 1.68g of iron is combined with 0.48g of
oxygen. What is the empirical formula of the new compound
2. (b) Confirm: 2.70g of aluminium is combined with 10.65g
of chlorine. What is the empirical formula of the new
3. Challenge: Saltpetre is a potassium salt. It is 13.9%
nitrogen ,38.6% potassium and 47.5% oxygen. What is its
empirical formula?

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