Answer
$472.08\ g/mol$
Work Step by Step
Freezing point depression: $-0.197^{\circ}C$
Molality: $\Delta T_f=K_fm\rightarrow m=-0.197^{\circ}C\div -1.86^{\circ}C/m=0.106\ mol/kg$
Number of moles: $0.106\ mol/kg\times (50.0/1000)kg=5.30\times10^{-3}\ mol$
Molar mass: $2.5\ g\div 5.30\times10^{-3}\ mol=472.08\ g/mol$
