In acetylation, each hydroxyl group reacts:
R-OH + (CH₃CO)₂O → R-O-CO-CH₃ + CH₃COOH.
Each -OH is converted to -OCOCH₃, resulting in a net mass increase of 42 g/mol per hydroxyl group (addition of acetyl group mass 43, removal of hydrogen mass 1).
Given: Original molar mass = 122 g/mol, Product molar mass = 290 g/mol
Total mass increase = 290 - 122 = 168 g/mol
Number of hydroxyl groups = 168 ÷ 42 = 4