(a)
Ethyl isocyanide on hydrolysis in acidic medium gives methanoic acid and ethyl amline salt
$C_{2}H_{5}NC + H_{2}O\overset{\ \ H + \ \ }{\rightarrow}HCOOH\ \ \ \ \ + C_{2}H_{5}NH_{2}$
methanoic acid
$C_{2}H_{5}NH_{2} + H^{+} \rightarrow \ C_{2}H_{5}NH_{3}^{+}$
Ethylamine salt