$Pb-Bi$ phase diagram is given below.

$x_{B1}$

for given temperatures, i$.$e$.,T_1$ and $T_2$

a$)$ Schematically plot the variation of Gibbs free energy vs$.$ composition $50$ pts

b$)$ Schematically plot the variation of activity vs$.$ composition $50pts$

Note: All answers must be shown on either a millimetric paper or a checkered paper