1000g of 1 molal aqueous solution of sucrose is cooled and maintained at $-{3.72}^{\circ }\text{C}$ . Calculate the weight of ice (in grams) that will separate out. 
($K_f$ for water$=1.86{}^{\circ }\text{C/molal}$  )

1342 g solution contains 342g sucrose 
1000g solution contains  $\frac{1000\times 342}{1342}=254.84\text{\hspace{0.17em}g\hspace{0.17em}sucrose}$
Weight of water present  $=1000-254.84$
                                                $=745.16\text{\hspace{0.17em}g}$
 $\Delta T_f=k_f.m$
 $3.72=1.86\times m$
 $m=2$
For molality to double, half the water in solution should separate out as ice.
i.e.  $\frac{745.16}{2}=372.58g$