This because of the general formula of alkenes being CnH 2n and since there is a factor of n it simplifies to CH 2. For instance we cannot say the exact number of Na and Cl in a NaCl crystal. The empirical formula for all alkene is CH 2.
It is basically the ratio for each of the components.
In this example we are calculating the empirical formula for mass composition. Glucose has a molecular formula of C 6 H 12 O 6. For example the empirical formula of ethane is CH3 because both numbers in its molecular formula can be simplified by dividing by 2. The empirical formula of a compound is the simplest whole number ratio of atoms of each element in the compound.