A compound with molecular formula C _ 4 H _ 10 O _ 3 in converted...

In acetylation, replacement of H of $O - H$ occurs by $C{H}_{3}CO -$ group. $- O - H + Cl \overset{O}{\overset{| |}{C}} - C{H}_{3} \to - OCOC{H}_{3} + HCl$

i.e., H atom of mass 1 amu is lost and an acetyl group of mass 43 amu is added. Thus, there is a net gain of 43 - 1 = 42 amu for every acetyl group introduced. Mass of difference of final product and original compound is 190 - 106 = 84.

Hence, number of $- OH$ groups $= \dfrac{84}{42} = 2$ .