Which of the following aqueous solutions will have the highest freezing point?

$17.4\% \ (W/V)$ solution of potassium sulphate (mol. $wt. = 174$) is isotonic with $4\% \ (W/V)$ aqueous solution of $NaOH$. If $NaOH$ is $100\%$ ionised,the degree of ionisation of potassium sulphate is $........... \ \%$.

Which of the following has the lowest freezing point?

The van't Hoff factor for $Ca(NO_3)_2$ is ...... .

An aqueous solution is prepared by dissolving $0.1 \ mol$ of an ionic salt in $1.8 \ kg$ of water at $35^{\circ} C$. The salt remains $90 \%$ dissociated in the solution. The vapour pressure of the solution is $59.724 \ mm \ Hg$. Vapor pressure of water at $35^{\circ} C$ is $60.000 \ mm \ Hg$. The number of ions present per formula unit of the ionic salt is . . . . . .

The measured osmotic pressure of a solution prepared by dissolving $17.4 \ mg$ of $K_2SO_4$ in $2 \ L$ of water at $27^{\circ} C$ is $3.735 \times 10^{-3} \ bar$. The van't Hoff factor is $(R = 0.083 \ L \ bar \ K^{-1} \ mol^{-1}$; atomic weights $K = 39, S = 32, O = 16)$.