Question

convert benzoic acid into benzoic anhydride class 12th

Answer 1

Phosphorous pentoxide ($P_{2} O_{5}$) is a dehydrating agent. It is used for various purposed. Here, it  is used to convert Benzoic acid into its acid anhydride which is Benzoic anhydride. 

Observation
When Phosphorous pentoxide is added to Benzoic acid, a white substance is formed which is used in preparation of benzoic esters( which proves it is Benzoic anhydride)

Reaction
$C_{7}H_{6}O_{2}$ + $P_{2} O_{5}$ ⇒ $C_{14}H_{10}O_{3}$