Answer
$406.35\ g/mol$
Work Step by Step
B.p. elevation: $61.93-61.70=0.23^{\circ}C$
Molality: $\Delta T_b=K_bm\rightarrow 0.23^{\circ}C=3.63^{\circ}C\times m\rightarrow m=0.063\ mol/kg$
Number of moles: $0.063\ mol/kg\times (25.0/1000)kg=0.0016\ mol$
Molar mass : $0.640\ g\div 0.0016\ mol=406.35\ g/mol$
