KBr is 80 % dissociated in solution.The freezing point of a 0.5 molal solutionis Kf of water is 1.86oc/m

For KBr i=1+αn-1                i= 1+0.82-1               i= 1.8 ∆Tf=iKfm =1.80×1.86×0.5=1.674 Tsolution=273-1.674=271.326 K