First let us get the molecular mass of the compound C2H5O.
C has molar mass of 12 amu, H is 1 amu, while O is 16 amu, therefore:
C2H5O = 12 amu * 2 + 1 amu * 5 + 16 amu = 45 amu
So to get 90 amu, simply double all the elements, therefore:
C4H10O2
Let us assume that the given formula of elements is empirical formula where elements have simplest whole number ratio
The empirical mass = 2 X atomic mass of carbon + 5 X atomic mass of hydrogen + at mass of O
The empirical mass = 2 X 12 + 5 + 16 = 45
The ratio of molecular mass and empirical mass = 90 / 45 = 2
Molecular formula is obtained by multiplying the empirical formula with this ratio / factor
molecular formula = 2 X (C2H5O) = C4H10O2
