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Need to
· know formula of compound (or molecule),
· look up relative atomic masses of all
atoms in the substance,
· add together the relative atomic mass of
each atom.…read more

Slide 3

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Example 1
What is the relative formula mass of
water?
It contains
The formula
of water is H 2
O 2 x H atoms
+ 1 x O atom
So its relative formula mass =
2 x Ar of H + 1 x Ar of O =
(2 x 1) + (1 x 16) = 18…read more

Slide 4

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Example 2
What is the relative formula mass of
sodium hydroxide? It contains
The formula 1 x Na atom
is NaOH + 1 x O atom
+ 1 x H atom
So its relative formula mass =
1 x Ar of Na + 1 x Ar of O + 1 x Ar of H =
(1 x 23) + (1 x 16) + (1 x 1) = 40…read more

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Example 3
What is the relative formula mass of
sodium carbonate? It contains
The formula 2 x Na atoms
is Na 2
CO 3 + 1 x C atom
+ 3 x O atoms
So its relative formula mass =
2 x Ar of Na + 1 x Ar of C + 3 x Ar of O =
(2 x 23) + (1 x 12) + (3 x 16) = 106…read more

Slide 6

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Example 4
What is the relative formula mass of
calcium hydroxide? It contains
The formula 1 x Ca atom
is Ca(OH)2 + 2 x O atoms
+ 2 x H atoms
So its relative formula mass =
1 x Ar of Ca + 2 x Ar of O + 2 x Ar of H =
(1 x 40) + (2 x 16) + (2 x 1) = 74…read more

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