$=\,\frac {No.\, of\, particles\, after \,association\, or \,dissociation}{No.\, of\, particles\, before\, association\, or \,dissociation}$

For               $M{X_2}\, \rightleftharpoons \,{M^{2 + }}\, + \,2{X^ - }$

$t\,=\,0$             $1$              $0$              $0$

at.eq.                  $1-\alpha $       $\alpha $       $2\alpha $

Total no. of particles

$=\,1\,-\,\alpha + \alpha +2\alpha \,=\,1+2\alpha $

$\therefore $ $i\,=\,\frac {1+2\alpha }{1}$ $=\,2$

$\therefore \alpha \,=\,0.50$ or $50\%$