What is the product formed when acetaldehyde reacts with $PCl_5$?

Addition of hydrogen cyanide $(HCN)$ to carbonyl compounds is:

Identify the symmetrical ketone from the following compounds.

Reduction of which compound gives an optically active alcohol?

The major product of the following reaction is

$HCHO$ was treated with a reagent $X$. The product formed upon hydrolysis in the presence of an acid gave $C_{2}H_{5}OH$. The reagent $X$ is