Identify compound $A$ used in the following reaction: Benzoic acid $\xrightarrow[\Delta]{A}$ Benzoyl chloride + Phosphorous oxychloride + Hydrogen chloride

$Ph-CO-OH + CH_3-O^{18}-H \xrightarrow{H^{+}} (X) + H_2O$. Identify $(X)$.

When carboxylic acid reacts with organolithium reagents to give ketones,side reactions sometimes occur. For example,
$HOCH_2CH_2CH(CH_3)CH_2CH_2COOH$ $\xrightarrow[(x) \, CH_3Li]{tetrahydro\,furan}$ $\xrightarrow[NH_4Cl]{H_2O}$ $\underset{63\%}{\text{Compound } A: HOCH_2CH_2CH(CH_3)CH_2CH_2COCH_3}$ + $\underset{37\%}{\text{Compound } (B)}$
The value of $(x)$ in the above reaction is:

An optically active compound $X$ has molecular formula $C_4H_8O_3$. It evolves $CO_2$ with $NaHCO_3$. $X$ reacts with $LiAlH_4$ to give an achiral compound. $X$ is

Arrange the following in decreasing order of their acidic strength: $C_6H_5COOH$,$FCH_2COOH$,$NO_2CH_2COOH$. Give an explanation for the arrangement.

Which of the following has the most acidic hydrogen?