The molecular mass of the compound is 90 amu. what is the molecular formula for c2h5o?

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