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)
c)
b)
a)
b)
c)
d)
d)
water, H2O
ethene, C2H4
butene, C4H8
propanone, C3H6O
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
Imagine we had a tiny balance and we could weigh compounds. If
we put a carbon dioxide molecule on our balance, how much would
you expect it to weigh? The formula of carbon dioxide is CO2, so
the molecule would weigh the same as one carbon atom plus two
oxygen atoms. To calculate the relative molecular mass of a
molecule, all you have to do is add up the relative atomic masses
of all the atoms.
accurate relative atomic
masses for doing calculations
(the ones you might see on a
detailed Periodic Table).
Instead, we use values which
are rounded up or down to
the nearest whole number.
One exception to this rule is
chlorine; we use 35.5 for the
mass of a chlorine atom.
Example 2
Calculate the RMM of carbon dioxide, CO2.
RMM (CO2) = RAM (C) + 2 x RAM (O)
= 12 + 32
= 44
Example 3
If you crystallize out a solution of a salt, such as
magnesium sulphate, molecules of water sometimes
get trapped in the crystal lattice. This is called water
of crystallization. For each MgSO4 unit, seven water
molecules are trapped. However, different salts trap
different numbers of water molecules.
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