Isomers are compound having same molecular formula but different structure formula
The molecular formula of butanoic acid and ethyl acetate is C4H8O2.
The structure formula of butanoic acid is CH3CH2CH2COOH, while structural formula of ethyl acetate is CH3COOCH2CH3.
Thus, both butanoic acid and ethyl acetate have same number of constituent atoms, therefore they have same atomic mass. Thus, molecular mass of ethyl acetate is 88.1 amu.