1. Relative Molecular Mass
Example One
This is a methane molecule, CH4. How many of each type
of atom does it have?
It has the formula CH4, which means that it contains one
carbon atom and four hydrogen atoms.
1. How many of each type of atom do the following
molecules have?
a) b)
a) water, H2O
b) ethene, C2H4
c) butene, C4H8
d) propanone, C3H6O
c) d)
The Periodic Table is a list of atoms in order of increasing mass. The first atom, hydrogen
weighs one atomic mass unit, carbon weighs 12, oxygen 16, and lead, near the end of the
periodic table weighs 207. We don’t bother using the
accurate relative atomic
Imagine we had a tiny balance and we could weigh compounds. If masses for doing calculations
(the ones you might see on a
we put a carbon dioxide molecule on our balance, how much would detailed Periodic Table).
you expect it to weigh? The formula of carbon dioxide is CO2, so Instead, we use values which
the molecule would weigh the same as one carbon atom plus two are rounded up or down to
the nearest whole number.
oxygen atoms. To calculate the relative molecular mass of a One exception to this rule is
molecule, all you have to do is add up the relative atomic masses chlorine; we use 35.5 for the
mass of a chlorine atom.
of all the atoms.
Example 2
Calculate the RMM of carbon dioxide, CO2.
If you crystallize out a solution of a salt, such as
RMM (CO2) = RAM (C) + 2 x RAM (O)
magnesium sulphate, molecules of water sometimes
= 12 + 32 get trapped in the crystal lattice. This is called water
= 44 of crystallization. For each MgSO4 unit, seven water
molecules are trapped. However, different salts trap
different numbers of water molecules.
Example 3
Calculate the RFM of hydrated magnesium sulphate, MgSO4.7H2O
RMM (MgSO4.7H2O) = RAM (Mg) + RAM (S) + 4 x RAM (O) + 14 x RAM (H) + 7 RAM (O)
= 24 + 32 + 64 + 14 + 112
= 246

2. Calculate the RMM of the following covalent compounds:
a) oxygen gas, O2
b) ozone gas, O3
c) hydrogen sulfide gas, H2S
d) ammonia gas, NH3
e) benzene, C6H6
f) octane, C8H18
g) paracetamol, C8H9O2N
h) glucose, C6H12O6
i) stearic acid, C18H36O2
j) adenosine triphosphate (ATP), C10H11O13N5P3
3. Calculate the RFM of the following ionic compounds:
a) sodium chloride, NaCl
b) magnesium oxide, MgO
c) magnesium hydroxide, Mg(OH)2
d) potassium cyanide, KCN
e) sulphuric acid, H2SO4
f) ammonium carbonate, (NH4)2CO3
g) potassium hexacyanoferrate(II), K4Fe(CN)6
h) hydrated iron(III) oxide (rust) Fe2O3.3H2O
i) hydrated copper sulfate, CuSO4.5H2O
j) hydrated potassium aluminium sulfate (alum), KAl(SO4)2.12H2O
Answers
1. How many of each type of atom do the following molecules have?
(a) two hydrogen atoms and one oxygen atom, (b) two carbon atoms and four hydrogen atoms, (c) four carbon atoms and eight
hydrogen atoms, (d) three carbon atoms, six hydrogen atoms and one oxygen atom
2. Calculate the RMM of the following covalent compounds:
(a) 32, (b) 48, (c) 34, (d) 17, (e) 78, (f) 114, (g) 151, (h) 180, (i) 284, (j) 502
3. Calculate the RFM of the following ionic compounds:
(a) 58.5, (b) 40, (c) 58, (d) 65, (e) 98, (f) 96, (g) 368, (h) 214, (i) 250, (j) 474