A compound being analysed has the percentage composition by mass Ca = 13.9%, W = 63.9%, O = 22.2%. Calculate the empirical formula of this compound

Using rows to represent mathematical operations and a column for each compound we can draw ourselves a table:

                             Ca       W      O
%                         13.9    63.9   22.2
Atomic Mass (Ar) 40       184    16
% / (Ar)                 0.348  0.347 1.43
Ratio                    1    :     1   :   4

This produces the empirical formula Ca₁W₁O₄ or more commonly written CaWO₄. We retrieve the atomic mass from our periodic table and then divide teh percentage by mass given in the question for each element by its atomic mass. We then take the smallest of these numbers and divide all the other % / (Ar) by it to produce a ratio, it doesn't matter if they aren't exact, they just have to be close to a whole number. This ratio is the ratio of elements in the empirical formula.