Question

Benzoic acid dissolved in benzene shows a molecular mass of ________$g/mol$
A. 122
B.244
C. 366
D. 488

Answer 1

Hint: The answer to this question includes the concept of reaction of benzoic acid with benzene that is benzoic acid forms a dimer where there is association through intermolecular hydrogen bonding and thus the observed mass will be twice.

Complete step by step answer:
In the classes of organic chemistry, we have studied about the dimerisation reactions and also several related reactions.
Now, we shall see what happens when benzoic acid is dissolved in benzene.
- Benzoic acid is a white solid compound with the formula $C_6{H}_{5}COOH$ and is the simplest aromatic compound.
- This compound when put into benzene solution, it dissolves due to the formation of intermolecular hydrogen bonding and thus this leads to the formation of a dimer.
- Benzoic acid is also known as benzene carbonic acid and the name is derived from gum benzoin which was its only source for a long time and this benzoic acid occurs naturally in many plants and serves as an intermediate in the biosynthesis of many secondary metabolites.
Thus, the molecular weight of this compound will be double that of the benzoic acid because of the formation of dimer and therefore the calculation will be as shown below,
$${(C_6{H}_{5}COOH)}_2=2\times 122=244g/mol$$
[Since, molecular weight of benzene = 122 g/mol]

Therefore, the correct answer will be option B. 244.

Note: Dimer is an oligomer that consists of two monomer units joined by the bonds which can either be a strong bond por can also be a weak bond or it can also be covalent or intermolecular bonds in nature. Homodimer is the one where monomers are the same and heterodimer is the one where monomers are different.