Here, values at different time,

                   $\begin{array}{cc}A{B}_2⟶ A^{2+}\mathit{ }\mathit{ }+& 2B^{-}\end{array}$

$\begin{array}{cccc}t=0 & 1& 0& 0\end{array}$

$t=t\mathit{ }\mathit{ }\mathit{ }\mathit{ }\mathit{ }\mathit{ }1-\alpha \mathit{ }\mathit{ }\mathit{ }\mathit{ }\mathit{ }\mathit{ }\mathit{ }\mathit{ }\alpha \mathit{ }\mathit{ }\mathit{ }\mathit{ }\mathit{ }\mathit{ }2\alpha$

$\begin{array}{l}{n}_T=1+2\alpha \\ i=1+2\times 0.8=2.6\\ \mathrm{\Delta }{T}_b=i{K}_{b}m=2.6\times 5\times 1=1.3\\ T_b=373+1.3=374.3 \mathrm{K}\end{array}$