Question

Which of the following are isomers?
A. Ethanol and ethoxyethane
B. Methanol and methoxy methane
C. Propanoic acid and ethyl acetate
D. Propanal and Acetone

Answer 1

Hint: Compounds which contain exactly the same number of atoms or have the same empirical formula but have different arrangements of atoms are called isomers. In other words, isomers have the same molecular formula and hence the same molar masses but different structures or arrangement of atoms.

Complete answer:
For identifying isomers between a pair of compounds, the molecular masses of both the compounds can be compared.
So, Ethanol with chemical formula ${C_2}{H_5}OH$ consists of two carbon atoms of mass 12 gram each and six hydrogen atoms of 1 gram each and one oxygen atom of 16 grams. Therefore, the molecular mass of Ethanol is
$(2)(12) + (6)(1) + 16 = 46{\text{ grams}}$
Hence, the molecular mass of Ethanol is 46 grams.
Now, Ethoxy ethane with chemical formula ${C_2}{H_5}O{C_2}{H_5}$ consists of four carbon atom of 12 grams each along with ten hydrogen atom of 1 gram each and one oxygen atom of 16 grams. Therefore, the molecular mass of Ethoxy ethane is
$(4)(12) + (10)(1) + 16 = 74{\text{ grams}}$
Hence, the molecular mass of Ethoxy ethane is 74 grams.
Since, the molecular mass of Ethanol and Ethoxy ethane are not same. Therefore, Ethanol and Ethoxy ethane are not isomers.
 Methanol with chemical formula $C{H_3}OH$ consists of one carbon atom of 12 grams along with four hydrogen atom of 1 gram each and one oxygen atom of 16 grams. Therefore, the molecular mass of Methanol is
$(1)(12) + (4)(1) + 16 = 32{\text{ grams}}$
Hence, the molecular mass of Methanol is 32 grams.
Now, Methoxy methane with chemical formula $C{H_3}OC{H_3}$ consists of two carbon atom of 12 grams each along with six hydrogen atom of 1 gram each and one oxygen atom of 16 grams. Therefore, the molecular mass of Methoxy methane is
$(2)(12) + (4)(1) + 16 = 46{\text{ }}grams$
Hence, the molecular mass of Methoxy methane is 46 grams.
Since, the molecular mass of Methanol and Methoxy methane are not same. Therefore, Methanol and Methoxy methane are not isomers.
Propionic acid with chemical formula $C{H_3}C{H_2}COOH$ consists of three carbon atom of 12 grams each along with six hydrogen atom of 1 gram each and two oxygen atom of 16 grams. Therefore, the molecular mass of Propionic acid is
$(3)(12) + (6)(1) + (2)(16) = 74{\text{ grams}}$.
Hence, the molecular mass of Propionic acid is 74 grams.
Now, Ethyl acetate with chemical formula $C{H_3}COO{C_2}{H_5}$ consists of four carbon atom of 12 grams each along with eight hydrogen atom of 1 gram each and two oxygen atom of 16 grams. Therefore, the molecular mass of Ethyl acetate is
$(4)(12) + (8)(1) + (2)(16) = 88{\text{ grams}}$
Hence, the molecular mass of Ethyl acetate is 88 grams.
Since, the molecular mass of Propionic acid and Ethyl acetate are not same. Therefore, Propionic acid and Ethyl acetate are not isomers.
 Propanal with chemical formula $C{H_3}C{H_2}CHO$ consists of three carbon atom of 12 grams each along with six hydrogen atom of 1 gram each and one oxygen atom of 16 grams. Therefore, the molecular mass of Propanal is
$(3)(12) + (6)(1) + 16 = 58{\text{ grams}}$
Hence, the molecular mass of Propanal is 58 grams.
Now, Acetone with chemical formula \[C{H_3}COC{H_3}\] consists of three carbon atom of 12 grams each along with six hydrogen atom of 1 gram each and one oxygen atom of 16 grams. Therefore, the molecular mass of Ethoxy ethane is
$(3)(12) + (6)(1) + 16 = 58{\text{ grams}}$.
Hence, the molecular mass of Acetone is 58 grams.
Since, the molecular mass of Propanal and Acetone are same. Therefore, Propanal and Acetone are isomers.
So the correct answer is option D.

Note:
Molecular mass can be defined as the sum of the atomic masses of each nuclide present in the molecule. Whereas, the molar mass can be defined as the sum of the standard atomic weights of all the constituent particles in a compound.  Relative atomic mass and molecular values are dimensionless.