$i\, = \,\frac{{\Delta {T_f}}}{{{K_f}\, \times \,m}}\, = \,\frac{{1.91}}{{1.86\, \times 1}}\, = \,1.02$

For $\mathop {HI}\limits_{(1 - \alpha )} \, \rightleftharpoons \,\mathop {{H^ + }}\limits_\alpha  \, + \,\mathop {{I^ - }}\limits_\alpha  $

$1 - \,\alpha \, + \alpha \, + \,\alpha \, = \,i\, = \,1.027$

$1\, + \,\alpha \, = \,1.027$

$\alpha \, = \,0.027$ Or $2.7\,\% $