How to calculate relative formula mass or relative
molecular mass RFM/RMM or Mr
If all the individual atomic masses of all the atoms in a formula are added together you have
calculated the relative formula mass (for ionic compounds e.g. NaCl = 58.5)or molecular mass (for
covalent elements e.g. N2 = 28 or compounds e.g. C6H12O6 = 180). To be honest, the term relative
formula mass can be used with any compound whether it be ionic or covalent - it just seems not
quite correct to talk about the molecular mass of an ionic compound when it doesn't consist of
molecules!
The shorthand Mr can be used for the formula of any element or compound. Whereas relative
atomic mass only applies to a single atom but anything with at least two atoms requires the term
relative formula mass or relative molecular mass.
The most common error is to use atomic/proton numbers instead of atomic masses,
unfortunately, except for hydrogen, they are different!
Examples of formula/molecular mass calculations:
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Recap: Molecular/formula mass = total of all the atomic masses of all the atoms in the
molecule/compound.
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Example 2.1: the diatomic molecules of the elements hydrogen H2 and chlorine Cl2
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o
relative atomic masses, Ar: H = 1, Cl = 35.5
o
Formula masses, RMM or Mr, are H2 = 2 x 1 = 2, Cl2 = 2 x 35.5 = 71 respectively.
Example 2.2: the element phosphorus consists of P4 molecules.
o
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RMM or Mr of phosphorus = 4 x its atomic mass = 4 x 31 = 124
Example 2.3: The compound water H2O
o
relative atomic masses are H=1 and O=16
o
RMM or Mr = (1x2) + 16 = 18 (molecular mass of water)
Example 2.4: The compound sulphuric acid H2SO4
o
relative atomic masses are H=1, S=32 and O=16
o
RMM or Mr = (1x2) + 32 + (4x16) = 98 (molecular mass of sulphuric acid)
Example 2.5: The compound calcium hydroxide Ca(OH)2 (ionic)
o
relative atomic masses are Ca=40, H=1 and O=16
o
RMM or Mr = 40 + 2 x (16+1) = 74
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Example 2.6: The ionic compound aluminium oxide (Al3+)2(O2-)3 or just plain Al2O3
o
relative atomic masses are Al = 27 and O = 16
o
so the formula mass RFM or Mr = (2 x 27) + (2 x 16) = 102
Example 2.7: calcium phosphate is also ionic but a more tricky formula to work out!
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o
(Ca2+)3(PO43-)2 or Ca3(PO4)3
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atomic masses: Ca = 40, P = 31, O =16
o
RFM or Mr = (3 x 40) + 3 x {31 + (4 x 16)} = (120) + (3 x 95) = 405