In acetylation reaction H atom of an (−OH) group is replaced by an acetyl group (−COCH3). This results increase in molecular mass by [(12+16+12+3×1)−1], that as, 42u. Increase in molecular mass =84u ∴‌‌ Number of −OH groups =‌
84u
42u
=2 The carbohydrate must be aldotriose as it contains two alcoholic groups.