Naturally occurring RAM values and atomic numbers for all elements can be found on a Periodic Table. Make sure you know which is which – some Periodic Tables have atomic number above element symbol, and some have it below. Atomic number is always less than RAM.
Masses of molecules
Th e mass of one molecule is obtained by adding up the atomic masses in that molecule.
E.g.
• N2: 2 x N = 2 x 14u = 28u
• CH4 (methane): C + (4 x H) = 12u + (4 x 1u) = 16u
• C8H10N4O2 (caffeine): (8 x C) + (10 x H) + (4 x N) + (2 x O) = (8 x 12u) + (10 x 1u) + (4 x 14u) + (2 x 16u) = 194u
Relative Molecular Mass
Relative molecular mass (R.M.M.) is obtained by dividing the masses by 1/12 of the mass of a Carbon 12 atom (i.e. by 1u).
Therefore R.M.M. of methane = 16 and R.M.M. of caffeine = 194. Strictly, as ionic compounds do not exist as molecules, we should refer to the Relative Formula Mass (RFM) of an ionic compound, not the RMM. Calculate the RMM (or RFM) for the following compounds:
• Ethanol, C2H5OH
• Potassium bromide
• Magnesium chloride
• Copper sulphate
• Water
