Which acid has the least $pK_a$ value?

In the Kolbe electrolysis of sodium propanoate,the products $X$ and $Y$ are formed at respective electrodes. What are $X$ and $Y$?

Identify the compounds $A$,$B$,and $C$ in the following reaction sequence:
$CH_3-Br$ $\xrightarrow{Mg, \text{Ether}} [A]$ $\xrightarrow[(ii) \text{Water}]{(i) CO_2} [B]$ $\xrightarrow{CH_3OH, H^+} [C]$

Identify the products of the following reaction:
$C_6H_5COOC_2H_5 \xrightarrow{\text{dil. } H_2SO_4} ?$

What is the correct order of acidic strength?

Number of compounds from the following which will not dissolve in cold $NaHCO_3$ and $NaOH$ solutions but will dissolve in hot $NaOH$ solution is $........$.