Explanation

(a)
In Hofmann method, a mixture of primary, secondary and tertiary amines is treated with diethyloxalate, when primary amine forms solid oxamide, secondary amine forms a liquid oxamic ester whereas tertiary amine remains unaffected.
$\left( COOC_{2}H_{5} \right)_{2} + 2HNHR \rightarrow (CONHR)_{2} + 2C_{2}H_{5}OH$
Diethyl oxalate 1${^\circ}$ amine solid
$CONR_{2}$
$\left( COOC_{2}H_{5} \right)_{2} + HNR_{2} \rightarrow |$
$COOC_{2}H_{5} + C_{2}H_{5}OH$
2${^\circ}$amine liquid
$\left( COOC_{2}H_{5} \right)_{2} + NR_{3} \rightarrow$No reaction
3${^\circ}$ amino