An unknown compound (A) having molecular formula  $C_6{H}_{12}{O}_6$ on acetylation gives acylated product whose molecular weight is 390. How many number of hydroxyl groups present in A?

$R-OH\to R-OCOC{H}_3$ 
Acylation of one hydroxyl group results in increment of  $-COC{H}_2\left(or\right)\left(C_2{H}_{2}O\right)$
$C_6{H}_{12}{O}_6\to C_6{H}_{12}{O}_6+\left(C_2{H}_{2}O\right)\times 5=C_{16}{H}_{22}{O}_{11}$   (Molecular weight = 390)
$\begin{array}{l}{C}_6{H}_{12}{O}_{6}M.wt=180\\ \text{Difference }wt.=390-180=210\end{array}$
One  $C_2{H}_{2}O$ having molecular weight  $=42\times 5=210$
No. of groups added = 5