$1 - \alpha $    $\frac{\alpha }{2}$

$1 - 0.8$    $\frac{{0.8}}{2}$

$i\, = 1 - 0.8\, + \,\frac{{0.8}}{2}\, = \,0.6$

$\Delta {T_f}\, = \,{K_f}\, \times \,\,i\,\, \times \,\,m$ $ = \,5\,\, \times \,0.6\,\, \times \,\frac{X}{{122}}\, \times \,\frac{{1000}}{{30}}\, = \,2$ (Since $\Delta {T_f}\, = \,2$ )

$\therefore \,X\, = \,2.44\,g$