Relative Molecular Mass of Compounds
Compounds contain two or more different types of atoms that are chemically bonded together.
Compounds cannot be easily separated into their elements.
The physical and chemical properties of a compound are completely different from the
elements that form it. For example, table salt is the compound Sodium Chloride, which is made
from the elements Sodium and Chlorine.
Sodium is a metal that is soft, grey, and shiny while chlorine is a pale green gas. They
combine to form sodium chloride which is a white crystal.
The Relative Molecular Mass (RMM) is the sum of all the Relative Atomic Masses of the
elements on that compound.
Procedure for calculation:
Example 1: Calculate the RMM of Magnesium Oxide
1. Write the molecular formula of the compound
Molecular formula = MgO
2. Multiply the RAM of each atom by the number of atoms present
1 atom of Mg = 1 x 24 = 24
1 atom of O = 1 x 16 = 16
3. Add all numbers to get the RMM
RMM of MgO = 24 + 16 = 40
Example 2: Calculate the RMM of methane
Molecular formula = CH4
1 atom of C = 1 x 12 = 12
4 atoms of H = 4 x 1 = 4
RMM = 12 + 4 = 16
Practice:
Calculate the RMM of:
NaCl
Na2S2O3
Pb(NO3)2
Mass percent of an element in a compound