When you have both of these ( The periodic table of elements and the formula of your compound) you are able to calculate the R.M.M ( Relative atomic mass) of that compound.
For example the formula of a NaCl ( Table salt ) has the elemnt Na and Cl.
We look at the atomic mass of both of these compounds
Na - 23
Cl - 35.5
R.M.M = 23 +35.5 = 58.5
Hope this helps :).
