For the dissociation of $Na_2SO_4$,how is the degree of dissociation $\alpha$ used to calculate the van't Hoff factor $(i)$?

At a constant temperature,the vapor pressures of aqueous solutions of $Na_2SO_4$,urea,$AlCl_3$,and $1.2 \, m \, KCl$ are approximately equal. The molalities of the aqueous solutions of $Na_2SO_4$,urea,and $AlCl_3$ are,respectively: (Assume $100 \%$ dissociation of the solute)

If a compound $AB$ dissociates to the extent of $75\,\%$ in an aqueous solution,the molality of the solution which shows a $2.5\, K$ rise in the boiling point of the solution is $......$ molal.
(Rounded-off to the nearest integer) $[K_{b} = 0.52\, K\, kg\, mol^{-1}]$

The freezing point of a solution prepared by adding $8.1 \ g$ of $HBr$ ($90\%$ ionized) to $100 \ g$ of water is $...... \ ^\circ C$. (Given $K_f$ for water $= 1.86 \ K \ kg \ mol^{-1}$)

The elevation in boiling point of a solution of $10 \ g$ of a binary electrolyte (of molecular mass $100$) in $100 \ g$ of water is $\Delta T_b$. Then,the value of $K_b$ for water is

The experimental molecular weight of an electrolyte will always be less than its calculated value because the value of Van't Hoff factor $i$ is