The compounds like amines and alcohols undergo acetylation with acetyl chloride. During this reaction, one proton of amine or alcohol is replaced with one acetyl group(CH3CO−).

The molar mass of the acetyl group is 43g/mol. The increase in molar mass due to one acetyl group is 43−1=42g/mol.
The molar mass of reactant is 108g/mol. The molar mass of product is 192g/mol. The change in mass is 84.
Hence, the number of acetyl groups =4284=2