An unknown compound (A), having molecular formula C₆H₁₂O₆ on acetylation, gives acylated product whose molecular weight is 390. How many number of hydroxyl groups are present in A?

Acetylation of an alcohol takes place in the following manner:
$\mathrm{R}-\mathrm{OH}\stackrel{\mathrm{acetylation}}{\to } \mathrm{R}-\mathrm{OAc}$
Thus, it is clear that each −OH group is replaced by a −OAc group. Now, the weight of an acetyl group (Ac) is 43 and the weight of the lost Hydrogen atom is 1. 

Upon completion of the reaction, for each −OH group present in a molecule, there is an increase of 43−1 = 42 units of weight.
Now, net increase in weight of the given molecule (390−180) = 210
Thus, total number of −OH groups in the original molecule is $\frac{390-180}{42}=5$